Abnormal Placentation: Placenta Previa, Vasa Previa, and Placenta Accreta.

Clinical Expert Series

Abnormal Placentation Placenta Previa, Vasa Previa, and Placenta Accreta Robert M. Silver,

MD

Placental disorders such as placenta previa, placenta accreta, and vasa previa are all associated with vaginal bleeding in the second half of pregnancy. They are also important causes of serious fetal and maternal morbidity and even mortality. Moreover, the rates of previa and accreta are increasing, probably as a result of increasing rates of cesarean delivery, maternal age, and assisted reproductive technology. The routine use of obstetric ultrasonography as well as improving ultrasonographic technology allows for the antenatal diagnosis of these conditions. In turn, antenatal diagnosis facilitates optimal obstetric management. This review emphasizes an evidence-based approach to the clinical management of pregnancies with these conditions as well as highlights important knowledge gaps. (Obstet Gynecol 2015;126:654–68) DOI: 10.1097/AOG.0000000000001005

PLACENTA PREVIA Background Placenta previa is defined as the placenta overlying the endocervical os. In the past, previas were characterized as complete, partial, and marginal depending on how much of the internal endocervical os was covered by the placenta. However, the use of transvaginal ultrasonography allows for precise localization of the placental edge and the cervical os. Accordingly, the nomenclature has been modified so as to eliminate the terms “partial” and “marginal.”1 Instead, all placentas overlying the os (to any degree) are termed previas and those near to but not overlying the os are termed low-lying.1 The precise relationship of the placenta and the os and the distance that it overlies or is away from the os From the University of Utah School of Medicine, Salt Lake City, Utah. The author thanks Susan Fox for assistance in the technical preparation of the manuscript and Anne Kennedy, MD, and Paula Woodward, MD, for assistance with the figures. Continuing medical education for this article is available at http://links.lww. com/AOG/A672. Correspondence: Robert M. Silver, MD, Department of Obstetrics and Gynecology, University of Utah School of Medicine, 30 North 1900 East, Room 2B308, Salt Lake City, UT 84132; e-mail: [email protected]. Financial Disclosure The author did not report any potential conflicts of interest. © 2015 by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. All rights reserved. ISSN: 0029-7844/15

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(eg, 3.2 cm over the os or 1.3 cm away from the os) should be reported for low-lying placentas. The incidence is estimated to be 1 in 200 pregnancies at term and varies throughout the world.2,3 The incidence appears to have increased in relationship to the increasing rate of cesarean deliveries. Indeed, there is a dose–response relationship between the number of previous cesarean deliveries and subsequent placenta previa.4 Other risk factors include previous spontaneous and elective pregnancy terminations and previous uterine surgery.5 As with cesarean delivery, the risk of previa increases with increasing numbers of prior pregnancy losses. Thus, although the pathophysiology of placenta previa remains uncertain, there appears to be an association between endometrial damage and uterine scarring and subsequent previa. Other risk factors for previa include increasing maternal parity, increasing maternal age, smoking, cocaine use, multiple gestations, and prior previa.6–8

Clinical Implications Placenta previa is associated with numerous adverse maternal and fetal–neonatal complications. Many of these are direct consequences of maternal hemorrhage. Indeed, women with placenta previa are at an approximately 10-fold increased risk of antepartum vaginal bleeding.9 The mechanism of bleeding is uncertain but appears to be attributable to separation

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of the placenta from the underlying decidua resulting from contractions, cervical effacement, cervical dilation, and advancing gestational age. In turn, bleeding is associated with an increased risk of need for blood transfusion, hysterectomy, maternal intensive care unit admission, septicemia, thrombophlebitis, and even maternal death.2,9–12 Fetal complications are primarily those associated with prematurity. A population-based U.S. cohort noted 55.6% of women with previas delivered after 37 weeks of gestation, 27.5% delivered between 34 and 37 weeks of gestation, and 16.9% delivered before 34 weeks of gestation.8 In turn, perinatal mortality rates are increased by threefold to fourfold.13 Perinatal morbidity also is substantial.

Diagnosis The “classic” presentation used to be painless vaginal bleeding in the third trimester. Of course, bleeding may be associated with abdominal pain, contractions, or both. Now, most previas are diagnosed by antenatal ultrasonography, which is considered diagnostic. Almost all women receiving prenatal care in high-income countries undergo routine screening using transabdominal ultrasonography. At that time, the placental location is systematically evaluated. In cases of suspected placenta previa on transabdominal ultrasonography, the patient should undergo transvaginal ultrasonography to more accurately delineate the relationship between the placenta and the endocervical os, which is considerably more accurate than transabdominal ultrasonography. For example, transvaginal ultrasonography will “reclassify” a diagnosis of low-lying placenta noted on transabdominal ultrasonography in the second trimester 26–60% of the time (Fig. 1).14,15 Transvaginal ultrasonography also improves the accuracy of the diagnosis in the third trimester (Fig. 2).16,17 Such an

approach greatly reduces the need for follow-up ultrasonograms to “exclude” placenta previa.15 Although there are theoretical concerns about increasing the risk of bleeding, the procedure is quite safe.16,18 Transabdominal ultrasonography serves as an excellent screen for placenta previa, and it is not necessary to perform transvaginal ultrasonography on all women.19 For example, using a cutoff of 4.2 cm between the placental edge and the os had 93.3% sensitivity, 76.7% specificity, and a 99.8% negative predictive value for previa.19 Translabial ultrasonography is another more accurate alternative to transabdominal ultrasonography.20 It is noteworthy that many placenta previas noted at midpregnancy during routine screening ultrasonograms will no longer be present by the time of delivery. The relationship between the cervix and the placenta changes over time with the placenta typically “moving away” from the cervix. Accordingly, only approximately 10–20% of previas at 20 weeks of gestation will remain previas in the late third trimester.21 The placenta does not truly “migrate.” Instead, it is thought that the placenta undergoes a process termed trophotropism, growing toward the area of the uterus with the best blood supply (typically the fundus). In turn, the portion of the placenta closest to the cervix regresses and atrophies. The result is that only a small proportion of previas noted early in pregnancy remains at term. The earlier in gestation a previa is noted, the higher the probability that it will “resolve” by the late third trimester.22 There is no evidence to guide the optimal time of subsequent imaging in pregnancies thought to have placenta previa. In stable patients it is reasonable to perform a follow-up ultrasonogram at approximately 32 weeks of gestation. This allows adequate time for “resolution” of low-lying placentas and avoids potentially unnecessary studies. It may be worthwhile to

Fig. 1. A. Apparent placenta previa on transabdominal ultrasonogram. Arrow shows placenta apparently overlying the endocervical opening. B. The same patient with no previa on transabdominal ultrasonogram after resolution of focal uterine contraction. Arrow shows the endocervical opening with no placenta. Silver. Placenta Previa, Vasa Previa, and Placenta Accreta. Obstet Gynecol 2015.


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Fig. 2. True placenta previa on transvaginal ultrasonogram. Silver. Placenta Previa, Vasa Previa, and Placenta Accreta. Obstet Gynecol 2015.

perform an additional study at 36 weeks of gestation (if the previa persists) to determine the optimal route and timing of delivery. There is no clear benefit from more frequent ultrasonograms (eg, every 4 weeks) in stable cases. Numerous factors influence the chances of the previa persisting until delivery such as prior cesarean delivery, the distance the placental edge overlies the os, and the thickness of the placenta edge.15,22 In cases of prior cesarean delivery and “resolved” previa or low-lying placenta, there is an increased risk for accreta and vasa previa, respectively. These topics are addressed subsequently.

Management The only safe and appropriate mode of delivery for placenta previa is by cesarean delivery. In the absence of accreta, this can usually be accomplished using a lower segment uterine incision. It is important for the operator to move quickly but carefully, because cutting through the placenta often is associated with increased maternal bleeding. When the placenta is transected, the umbilical cord should be quickly clamped after delivery to avoid excessive blood loss. High vertical incisions are justified in some cases to avoid the placenta, especially if childbearing is completed or in cases of preterm gestations or transverse lie. Preoperative ultrasonography to precisely determine placental location is often useful in determining the optimal place for the uterine incision. The

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operators should be experienced owing to increased risks and difficulty of the procedure.23 There is an increased risk of postpartum hemorrhage in the setting of previa, even without accreta. This often is attributable to diffuse bleeding at the placental implantation site in the lower uterine segment. In addition to uterotonics, measures such as “oversewing” of the placental bed, intrauterine balloon tamponade, and B-Lynch or other compression sutures may be helpful. Cesarean delivery for placenta previa can usually safely be performed using regional anesthesia, and general anesthesia is reserved for unstable patients, the need for hysterectomy, and other nonobstetric indications. In fact, general anesthesia may increase blood loss without improving safety in cases of placenta previa.24 Although cesarean delivery is universally accepted as the optimal approach if the placenta overlies the endocervical os, if the placenta is 2 cm or greater from the os, a trial of labor is appropriate and the risk of bleeding is acceptable.25,26 Conversely, if there is less than 1 cm between the edge of the placenta and the os, the chances of hemorrhage are so high as to warrant planned cesarean delivery.26,27 The optimal management of patients with between 1.0 and 2.0 cm between the os and the placenta is uncertain (Fig. 3). Success rates of vaginal delivery range between 76.5% and 92.9% in small series.27–29 It is reasonable to individualize the management of such patients. The optimal timing of delivery in “stable” cases of placenta previa also is controversial. The benefits of a planned delivery under optimal circumstances and before labor or bleeding must be weighed against the risks of prematurity. There are no quality data to guide management, and it is difficult to analyze retrospective cohorts because many studies do not differentiate between elective and emergent delivery. The risk of bleeding, labor, or bleeding and labor leading to the need for emergent delivery increases with increasing gestational age, and the risks of morbidity associated with prematurity decrease with advancing gestational age. A decision analysis concluded that the optimal strategy was planned delivery at 36 weeks of gestation.30 This issue is well-summarized in a review by Blackwell, who concluded that women with uncomplicated placenta previa should undergo scheduled late preterm birth by cesarean delivery between 36 0/7 and 37 0/7 weeks of gestation.31,32 There is no need to assess fetal maturity with amniocentesis. Earlier delivery may be warranted for those with meaningful vaginal bleeding, labor, or other comorbidities such as obesity or multiple prior cesarean deliveries.31




and gestational age after cerclage placement.38 However, another slightly larger trial showed no benefit to cerclage.39 Taken together, there is no clear benefit from the procedure in women with previa and it is not recommended.

VASA PREVIA Background

Fig. 3. “Low-lying” placenta on transvaginal ultrasonogram. The distance between the placental edge and the cervix is clearly visualized despite the fetal head overlying the cervix. D, distance. Silver. Placenta Previa, Vasa Previa, and Placenta Accreta. Obstet Gynecol 2015.

The mainstay of antepartum care is “expectant management.” Most women with asymptomatic previa (no bleeding or contractions) are managed as outpatients. Several,33,34 but not all,35 retrospective studies show similar outcomes with outpatient management at lower cost. In contrast, hospitalization is advised in most cases of vaginal bleeding or uterine contractions. One small, randomized controlled trial noted no difference in outcomes between patients with outpatient compared with inpatient care after an initial period of hospitalization and stabilization after a single episode of bleeding.36 It is difficult to standardize an approach to hospitalization for symptomatic previas given the potentially episodic nature of vaginal bleeding. Nonetheless, most experts advise hospitalization in women with multiple episodes of bleeding or in cases in which the patient has limited access to appropriate medical care.37 It is appropriate to consider corticosteroids to enhance fetal pulmonary lung maturity in all cases of previa; they should always be given in cases of expectant management after vaginal bleeding before 34 weeks of gestation. The use of tocolysis in women with previa is a matter of debate and the potential benefit of tocolysis in women with previa remains uncertain. Although often prescribed, the benefits of bed rest, pelvic rest, or reduced activity remain unproven. Cerclage was proposed to potentially stabilize the cervix and decrease bleeding. One small prospective trial noted an increase in birth weight


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Vasa previa is defined as fetal vessels that run through the fetal membranes, over or near the endocervical os, and unprotected by the placenta or umbilical cord (Fig. 4). It is uncommon and occurs in 1 in 2,500 to 1 in 5,000 pregnancies.40–42 The pathophysiology is uncertain, but there is some overlap with placenta previa. Risk factors include succenturiate or bilobed placenta, velamentous cord insertion, previa or lowlying placenta in the second trimester, and multiple gestation.41,43–45

Clinical Implications Undiagnosed vasa previa is associated with up to a 60% rate of perinatal mortality.41 This is the result of fetal–neonatal hemorrhage and exsanguination if fetal

Fig. 4. Vasa previa. Fetal vessels traverse the fetal membranes unsupported by Wharton’s jelly. Silver. Placenta Previa, Vasa Previa, and Placenta Accreta. Obstet Gynecol 2015.



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vessels tear in the process of spontaneous or artificial rupture of membranes and labor. Because the entire fetal blood volume is usually less than 100 mL/kg, clinically important bleeding can be rapid. Also, there is a theoretical risk of compromised blood flow to the fetus from compression and occlusion of fetal vessels by the presenting fetal part. Vasa previa also is associated with an increased risk of preterm birth and associated complications of prematurity. In most cases, this is the result of iatrogenic preterm birth in an effort to avoid stillbirth. The most important variable influencing fetal–neonatal outcome is prenatal diagnosis. Oyelese and colleagues40 noted a 97% survival rate in cases of prenatal diagnosis compared with only 44% when the diagnosis was made intrapartum in a cohort of 155 women with vasa previa. Gestational age at delivery is the only other variable associated with perinatal outcomes.41

Diagnosis Rarely, diagnosis may occur by palpation of fetal vessels or fetal tachycardia and sinusoidal pattern on fetal heart rate tracing. Because these approaches are unreliable and are associated with unacceptably high death rates, morbidity resulting from vasa previa was thought to be unavoidable. However, the condition is now frequently diagnosed by antenatal ultrasonography. It is standard to assess placental location, cord insertion, and number of placental lobes as part of a midpregnancy obstetric ultrasonography. When cases of low-lying placenta,

multilobed placenta, succenturiate lobed placenta, or velamentous cord insertion are noted, targeted screening for vasa previa should be performed using transvaginal ultrasonography and color Doppler. Typical findings include a linear tubular echolucent body overlying the os on gray scale ultrasonography. Color Doppler demonstrates flow through the structure and pulsed Doppler shows fetal vascular waveforms (Fig. 5). It is noteworthy that a “free loop” of cord can overlie the os and mimic a vasa previa. Accordingly, it is important to trace the insertion of the vessels and to demonstrate that they do not “move” with changing maternal position. Also, it may be helpful to use the Trendelenburg position to remove the fetal presenting part out of the pelvis. In addition to assessing if the vessel moves, this may allow better visualization of the vessels on ultrasonography without compression by the presenting part.46 It is likely that a combination of both transabdominal and transvaginal ultrasonography provides the best diagnostic accuracy.23 The performance characteristics of ultrasonography for the detection of vasa previa have been reported to be excellent.42,43,47,48 However, these studies may overestimate accuracy because they were conducted in specialty centers, the number of vasa previas was small, and the cohorts assessed were subject to bias. Power Doppler, three-dimensional ultrasonography, and magnetic resonance imaging all have been reported as beneficial modalities to diagnose vasa previa.49,50 However, it is unclear that they are superior to

Fig. 5. Vasa previa. Color Doppler demonstrating fetal vessels over the cervix. Pulse wave Doppler showing pulsatile flow in the vessels. Silver. Placenta Previa, Vasa Previa, and Placenta Accreta. Obstet Gynecol 2015.

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two-dimensional ultrasonography alone. It is noteworthy that although it is possible to diagnose vasa previa antenatally, some cases will be missed by competent centers.37

Management The optimal management of vasa previa remains controversial owing to a lack of quality data. It is generally accepted that delivery should occur by cesarean before the onset of labor of rupture of membranes. As with previa and accreta, the risk of perinatal death increases (as a result of increasing chances of labor, bleeding, and ruptured membranes), whereas risks of prematurity decrease with advancing gestational age. A decision analysis noted that scheduled delivery between 34 and 35 weeks of gestation is optimal with no need to assess fetal pulmonary maturity.51 Others also recommend 35–36 weeks of gestation.49 Urgent cesarean delivery should be accomplished in cases of vaginal bleeding with suspected vasa previa.23,52 Another area of uncertainty is the need for hospitalization. Inpatient management theoretically allows for timely delivery in cases of bleeding, rupture of membranes, or labor and could be lifesaving. Many authorities advise hospitalization at 30–32 weeks of gestation.53 Conversely, benefit remains unproven and outpatient care has been associated with excellent outcomes.53 The probability of bleeding in women with previas is less if the cervix is greater than 2.5 cm in length.54 Based on this observation as well as a lower probability of labor, women with normal cervical length are the best candidates for outpatient management. With suspected vasa previa, it is worthwhile to repeat the ultrasonography in the third trimester, because approximately 15% of apparent vasa previas will resolve by the late third trimester.55 As with previa, the optimal frequency of ultrasonograms for cases of suspected vasa previa is uncertain. If vasa previa is suspected at 20 weeks of gestation, follow-up ultrasonograms at 28–30 and 32–34 weeks of gestation are reasonable to confirm the persistence of the diagnosis before indicated preterm birth through cesarean delivery. There have been case reports of fetoscopic laser ablation of vasa previas.56 However, there are too few published cases to assess the risk-to-benefit ratio for this procedure.

PLACENTA ACCRETA Background Placenta accreta is defined as trophoblastic attachment to the myometrium without intervening


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decidua. If the trophoblast invades the myometrium, it is termed placenta increta, and if it invades through the myometrium beyond the serosa and into surrounding structures such as the bladder, it is termed a percreta. Often the term placenta accreta is used to refer to the entire spectrum of conditions including accreta, increta, and percreta as well as to cases of clinically apparent morbidly adherent placenta. In this article, placenta accreta refers to the spectrum unless specifically noted. In normal circumstances, trophoblast stops invading the uterus when Nitabuch’s (spongiosus) layer is reached in the decidua. In cases of accrete, this fails to occur, possibly as a result of damaged or deficient Nitabuch’s layer. The incidence appears to be dramatically rising in high-income countries and is thought to be the result of a similar increase in the rate of cesarean delivery. In the United States, the incidence increased from 1 in 30,000 pregnancies in the 1960s to approximately 1 in 2,500 pregnancies in a cohort from 1985 to 1994.57 This further increased to 1 in 533 pregnancies in a cohort from 1982 to 2002.58 Other more recent reports note a rate as high as 1 in 300 pregnancies, though precise data are lacking.59,60 Regardless of the incidence of accreta, it is clear that prior cesarean delivery and especially multiple cesarean deliveries are major risk factors.60–63 An Irish study noted an increasing rate of accretas specifically in women with prior cesarean deliveries.63 A large multicenter U.S. cohort study noted an increasing risk of placenta accreta with increasing numbers of cesarean deliveries.62 This was especially true for women with placenta previa and prior cesarean deliveries; in cases of placenta previa, the risk of accreta was 3%, 11%, 40%, 61%, and 67% for first, second, third, fourth, and fifth or more cesarean deliveries, respectively.62 It is noteworthy that the rate of accreta increases with increasing cesarean deliveries, even without placenta previa.62 Any surgery that damages or transects the endometrium increases the rate of subsequent accreta. These include uterine curettage, myomectomy, pelvic radiation, and endometrial ablation.64–66 Women with prior endometrial ablation are at particularly high risk of accreta. Other risk factors include smoking, in vitro fertilization, advanced maternal age, multiparity, previa, and a short interval between a prior cesarean delivery and subsequent pregnancy.58,67,68 It is likely that surgical technique plays a role in the pathophysiology of accreta, because rates vary throughout the world, and some but not all women with risk factors develop the condition.



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Clinical Implications Accreta is associated with considerable maternal and fetal morbidity and even mortality. Maternal complications are primarily the result of massive hemorrhage. Indeed, many cases are associated with “audible hemorrhage.” In turn, this leads to disseminated intravascular coagulation, multiorgan failure, the need for additional surgery including hysterectomy, thromboembolism, and even death. Median estimated blood loss in cohorts of accretas ranges from 2,000 to 7,800 mL.69 Similarly, the median number of units of blood transfused is five69 and many women require multiple units of blood and other blood products.69–73 In a series of 76 cases from Utah, 28% had disseminated intravascular coagulation.70 Maternal morbidity is common and 25–50% of patients are admitted to an intensive care unit.70,71 There is an increased risk of thromboembolism, pyelonephritis, pneumonia, adult respiratory distress syndrome, and renal failure.70,74 Infection also is common and may occur in the wound, abdomen, or vaginal cuff.70,75,76 Vesicovaginal fistula is rare but is a serious cause of morbidity.77 Finally, maternal death has been reported in up to 7% of cases.74 Most recent large series have lower rates of maternal death.69–73 However, maternal deaths are likely underreported. Surgical complications also are common, owing to the frequent need for hysterectomy, which can be technically difficult. The most frequent problem is injury to the bladder. However, it is difficult to assess the true rate of incidental cystotomy, because it is often performed intentionally to facilitate the surgery and to avoid worse injury. Ureteral injury has been reported in 10–15% of patients.75,76 Other less common complications include injury to the bowel, large vessels, and pelvic nerves. These morbidities are all more likely in cases of percreta. Finally, many women need repeat operations to control hemorrhage, treat infection, or address an injury to a pelvic structure. Neonatal complications are primarily the result of preterm birth. The average gestational age of delivery of accretas is typically 34–36 weeks of gestation, typically as a result of medically indicated preterm birth.70,71,73,78

zone also have been reported.79,80 In the second and third trimesters (gray scale ultrasonography) accretas are associated with the presence of placenta previa, loss of the normal hypoechoic retroplacental–myometrial zone, multiple vascular lacunae (Fig. 7), extension of the villi into the myometrium or beyond, interruption of the uterine serosa–bladder interface, and decreased retroplacental space (less than 1 mm).79 Doppler findings associated with accreta include turbulent lacunar blood flow, increased subplacental vascularity (Fig. 8), vessels bridging the placenta to the uterine margin, and gaps in myometrial blood flow.79,80 Placental lacuna and turbulent flow are the findings most consistently linked to accreta.79–81 Threedimensional power Doppler may improve accuracy but data are preliminary and the technique should be considered experimental.82

Box 1. Ultrasound Findings Suggesting Placenta Accreta Spectrum First trimester Gestational sac that is located in the lower uterine segment Multiple irregular vascular spaces noted within the placental bed Implantation of the gestational sac imbedded into the uterine window at the site of the prior cesarean delivery (“cesarean scar ectopic”) Second trimester Multiple vascular lacunae within the placenta Third trimester Loss of the normal hypoechoic retroplacental zone The presence of multiple vascular lacunae within placenta (Swiss cheese appearance) Abnormalities of the uterine serosa–bladder interface (interruption of the line, thickening of the line, irregularity of the line, and increased vascularity) Extension of the villi into the myometrium, serosa, or bladder Retroplacental myometrial thickness of less than 1 mm Turbulent blood flow through the lacunae on Doppler ultrasonography Increased subplacental vascularity Vessels bridging from the placenta to the uterine margin Gaps in myometrial blood flow

Diagnosis The mainstay of antenatal diagnosis is obstetric ultrasonography (Box 1). In some cases diagnosis is possible in the first trimester. Abnormalities include a cesarean scar ectopic pregnancy or a gestational sac that is implanted in the lower uterine segment (Fig. 6).79,80 Irregular vascular spaces within the placental bed and loss of the retroplacental–myometrial

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Reprinted from Silver RM, Fox KA, Barton JR, Abuhamad AZ, Simhan H, Huls CK, et al. Center of excellence for placenta accrete. Am J Obstet Gynecol 2015;212:561–8, with permission from Elsevier.




Fig. 6. Cesarean delivery scar from ectopic pregnancy in first-trimester ultrasonogram. If untreated, this often progresses to placenta accreta. Arrow shows gestational sac in the cesarean delivery scar, outside and superior to the uterine cavity. Silver. Placenta Previa, Vasa Previa, and Placenta Accreta. Obstet Gynecol 2015.

On balance, the sensitivity and specificity for second- and third-trimester ultrasonography for the identification of accreta are reported to be high and approximately 80–90%.79,80 However, these data may overestimate the accuracy of ultrasonography, because they are derived from referral centers with (often single) experts privy to the clinical history performing the ultrasonograms. The accuracy of ultrasonography for

the diagnosis of accreta was considerably less in a recent study wherein clinicians were blinded to the clinical status of the patients.81 Moreover, there was considerable variation among “experts” in the prediction of whether an accreta was present.83 Thus, the high predictive values reported previously may not be generalizable to the average medical center. Magnetic resonance imaging (MRI) also has been used to diagnose placenta accreta. It also is reported to have good accuracy for the prediction of the condition (Fig. 9).84–86 However, available studies are subject to even more bias than those using ultrasonography, because MRI is typically only used in patients at very high risk for accreta. In addition, it is not clear that MRI improves diagnostic prediction of accreta compared with obstetric ultrasonography.79,86 Taken together with the cost, lack of widespread availability, and lack of radiologists with expertise in using MRI to diagnose accreta, MRI is not recommended for routine use in suspected accreta.1 It may have a role as an adjunctive tool in difficult cases such as if the placenta is posterior or to assess invasion of adjacent organs in suspected percreta.79,86 It is unclear that serial ultrasonograms improve diagnosis or outcome once accreta is suspected. As with previa and vasa previa, there are no data to guide the optimal frequency of ultrasonograms. It is reasonable to consider follow-up studies at 28–30 and 32–43 weeks of gestation to confirm the diagnosis, precisely locate the placenta (which may aid delivery), and to assess possible bladder invasion.

Fig. 7. Placental lacunae (arrows) in a case of placenta accreta.

Fig. 8. Turbulent flow in the lacunae and increased vascularity between the placenta and bladder in a case of placenta percreta using color Doppler.

Silver. Placenta Previa, Vasa Previa, and Placenta Accreta. Obstet Gynecol 2015.

Silver. Placenta Previa, Vasa Previa, and Placenta Accreta. Obstet Gynecol 2015.


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Fig. 9. Magnetic resonance image demonstrating placenta percreta. A. Vessels in bladder wall (arrows). B. Percreta at bladder dome (arrow). Silver. Placenta Previa, Vasa Previa, and Placenta Accreta. Obstet Gynecol 2015.

Several maternal serum biomarkers have been proposed as being useful to predict accreta in women at risk. Although of interest, none are accurate enough to recommend routine use in screening for placenta accreta.

Management The optimal management of placenta accreta is unclear owing to a lack of randomized clinical trials. A recent survey of health care providers noted widely varied approaches to virtually every aspect of care for accretas.87 These included referral to subspecialists, timing of delivery, surgical techniques, the use of radiographic embolization of pelvic vessels, blood product replacement, and ureteral stents.87 In this section, evidence-based guidelines are highlighted and knowledge gaps and controversies outlined. One of the most important modifiers of clinical outcome is prenatal diagnosis of accreta. Several studies confirm decreased hemorrhage and other maternal complications in cases diagnosed antenatally rather than intrapartum.70–73 Prenatal diagnosis allows for optimal management, which typically includes planned cesarean hysterectomy before the onset of labor or bleeding. The hysterotomy (often fundal) is made with care to avoid transecting the placenta and no attempt is made to remove the placenta. Simply avoiding an attempt at placental removal can dramatically reduce the amount of bleeding as well as other complications.70–73 In addition, prenatal diagnosis allows for delivery to be arranged in a center that is best suited to care for women with accreta. Several recent studies demonstrate improved outcomes and fewer complications in cases managed at such centers.73,78,88,89 Such centers

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may be termed “centers of excellence” for placenta accreta.90 Features of such centers include multidisciplinary teams that are experienced with accretas and provide coordinated care. This may include specialists in maternal–fetal medicine, gynecologic surgery, gynecologic oncology, vascular trauma and urologic surgery, transfusion medicine, intensive care, neonatology, interventional radiology, and anesthesiology as well as specialized nursing staff and ancillary personnel. The specialty of the physicians is likely less important than experience and expertise in accreta. Ideally, multidisciplinary preoperative consultation will occur along with the use of a checklist to reduce errors (Box 2). A critical feature is a well-equipped blood bank with ample blood products, activated factor VII, and other alternative blood products. Team training and simulation can further enhance the effectiveness of the multidisciplinary team.90 Women with clinical or ultrasonographic risk factors for accreta should be referred to a center of excellence or imaging expert for evaluation of possible accreta. Delivery in a center of excellence is advised if there continues to be suspicion for accreta based on a combination of clinical and ultrasonographic characteristics.90 Occasionally placenta accreta will not be apparent until the time of laparotomy or cesarean delivery. It is still often possible to safely transport patients to appropriate centers under such circumstances. First, if an accreta is suspected in a stable patient at the time of laparotomy (as a result of placenta protruding through or hypervascularity noted in the lower uterine segment [Fig. 9]), it is appropriate to stop the procedure. At that time, additional preparations can be made (procuring




Box 2. Preoperative Checklist for Patients With Placenta Accreta • Consents signed including cesarean delivery, hysterectomy, cystoscopy, stent placement, anesthesia, arterial line, central line, blood transfusion, and repair or removal of damaged organs • History and physical examination done • Case scheduled in the “main” operating room • Anesthesia attending notified • Interventional radiology attending notified • Labor and delivery charge nurse notified • Other subspecialty surgeons notified as needed • Blood bank and transfusion medicine notified • Intensive care unit notified • Neonatal intensive care unit team notified • Blood typed and crossed (number of units: ____) • Recent complete blood cell count and prothrombin time, partial thromboplastin time, and international normalized ratio available • Massive transfusion protocol available • Cell saver available if needed • Ultrasonography available for placental mapping • Pelvic balloon for pressure or compression available • Sterile milk available for bladder instillation • Uterotonics available (Pitocin, Methergine, Hemabate, Misoprostol) • Cystoscopy equipment and ureteral stents available • Large-bore intravenous access • Rapid infusion equipment • Warmers for blood and patient • Venous thromboembolism prophylaxis • Point-of-care testing for hematocrit, etc • Antibiotics available Modified from protocols developed by Alfred Abuhamad, MD, and Karin Fox, MD, with permission. Data from Publications Committee, Society for Maternal-Fetal Medicine, Belfort MA. Placenta accreta. Am J Obstet Gynecol 2010;203:430–9.

blood, surgeons, equipment) before proceeding. Alternatively, it is reasonable to close the laparotomy and then safely transport the patient to a center of excellence for definitive management.90 If the patient is stable, it also is reasonable to transport the patient after a hysterotomy has been performed and closed. In the setting of unanticipated hemorrhage, it is necessary to proceed with resuscitation and operative intervention. If bleeding persists after hysterectomy, transport may be possible after stabilization with pelvic and abdominal packing. The optimal timing of delivery for accretas remains controversial. As with previa and vasa previa, the risks of prematurity must be balanced against the risk of unscheduled delivery in the setting of labor or bleeding. A decision analysis indicated that 34 weeks of gestation is the optimal time for planned delivery of


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women with placenta accreta.91 However, timing of delivery may be individualized based on the probability of accreta, whether there is a percreta, prior obstetric history (eg, preterm labor), uterine contractions, vaginal bleeding, cervical length, patient compliance, and access to care. Preterm premature rupture of membranes, bleeding, and contractions all are associated with unscheduled preterm delivery in women with accreta.92 It is noteworthy that most women without bleeding or contractions can have a safe and planned delivery at 36 weeks of gestation.92 Accordingly, it may be reasonable to individualize timing of delivery between 34 and 36 weeks of gestation in stable patients without bleeding or labor. Given the high probability of preterm birth, corticosteroids should be administered to all women with suspected placenta accreta. This can be done with the onset of bleeding or labor before 34 weeks of gestation or before planned delivery; for example, at 33.5 weeks of gestation. As with stable previa, there is no consensus or data to support or refute the benefits of antenatal hospitalization. Although of unproven efficacy, it is reasonable to hospitalize preterm accretas with bleeding or labor, especially if the patient is remote from an appropriate medical center. The optimal surgical approach to accreta remains controversial. Most authorities advise a planned cesarean hysterectomy with fundal hysterotomy to avoid compromising the placenta (Fig. 10). The surgery should be performed in an appropriately equipped and staffed operating room with point-of-care testing (eg, hematocrit or lactate). The abdominal incision should allow for easy performance of a difficult hysterectomy. This is typically a vertical incision, although a Cherney incision affords excellent access. Anesthesia is typically general with endotracheal intubation because the patient may become unstable as a result of hemorrhage. Large-bore intravenous access, pneumatic compression stockings, and possible access for hemodynamic monitoring are prudent. All of the measures pertinent to the optimal care of obstetric hemorrhage including a massive transfusion protocol, use of cell saver, and rapid infusion equipment also are appropriate.90,93 The use of ureteral stents with accretas also varies among clinicians.87 The use of ureteral stents also is controversial in nonaccreta gynecologic surgery. One study noted fewer ureteral injuries in cases using stents.70 However, the benefit of stents remains uncertain and use is best left to the preference of the surgeon. There also is debate about the routine use of pelvic devascularization. Advocates believe that using balloon catheters to occlude the uterine or internal



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Fig. 10. Placenta protruding through a prior cesarean delivery scar in a case of accreta. Silver. Placenta Previa, Vasa Previa, and Placenta Accreta. Obstet Gynecol 2015.

iliac arteries substantially decreases blood loss in cases of accreta and especially percreta.94,95 Typically balloons are placed but not inflated before initiating the surgery. They are then inflated after delivery of the neonate so as to not compromise fetal blood supply. However, the procedure has considerable risks such as arterial damage, occlusion, and infection.95–97 Also efficacy is unclear, because hypogastric artery ligation has not been shown to be effective with accretas70 and there is considerable collateral circulation to the uterus. Although prophylactic use of balloons is controversial, radiographic embolization of the hypogastric vessels remains an important strategy to reduce residual hemorrhage posthysterectomy, especially in stable patients with slow oozing without a clearly identified source of bleeding. Conservative management of accreta is another unresolved controversy. This usually refers to a strategy that intends to preserve the uterus. This is highly desirable because it preserves fertility and avoids a major and morbid operation. Typically this is accomplished by performing a fundal hysterotomy with care to avoid the placenta, ligating the cord close to the placenta without disrupting it, closing the hysterotomy, and allowing the placenta to resorb over time. Reasonably good outcomes have been reported using this approach.98 Despite these good outcomes, concerns remain about conservative management. The largest concern is that patients who underwent conservative management may comprise a different population than those who underwent planned cesarean hysterectomy. It is possible that some women with successful expectant management did not truly have placenta accreta or had mild focal

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accretas that pose less risk than those reported in other series. It is noteworthy that the French cohort of accretas had fewer prior cesarean deliveries, placenta previas, and ultrasonographic features of accreta than other cohorts.98 Also, several cases of severe morbidity have been noted with attempted conservative management.99 Others also have reported high rates of complications with conservative management100 and we are aware of several (unpublished) cases of attempted conservative management in which the patients experienced bleeding, infection, or bleeding and infection at unpredictable times remote from delivery. Accordingly, patients should be counseled regarding potential risks of conservative management. It is not clear that methotrexate enhances the rate of placental reabsorption. In theory, it may not be effective, because there are few actively dividing trophoblast at term. Given the risks and a lack of proven efficacy, it is not recommended. The term “conservative management” also is used to describe planned delayed hysterectomy, typically in cases of percreta. Hysterotomy is performed to deliver the neonate and the placenta is left in situ. After a variable period of time, and with or without a radiographic devascularization procedure, a hysterectomy is performed. In theory, this allows for involution of the uterus and placenta, decreased vascularity, and a less morbid surgery. Good outcomes have been reported in several cases using this approach.101 However, serious complications also have been reported, and it is unclear whether this strategy decreases or increases the risk of morbidity and mortality. Finally, another strategy for conservative management is to surgically remove a small portion of the uterus in the setting of focal accreta. This area can then be repaired and the patient is treated as though she had a classical cesarean delivery. Other strategies involve oversewing bleeding areas of the uterus or lower uterine segment in hopes of preserving the uterus. An example is the Triple P procedure, which involves resection of a portion of the uterus along with pelvic devascularization.102 In a follow-up study of 96 women with successful conservative management, eight had Asherman’s syndrome.103 Twenty-four women had 34 subsequent pregnancies, with 21 (62%) resulting in thirdtrimester deliveries.103 Recurrent accreta was noted in 6 of 21 (28.6%); of these, four of six were conservatively managed.103 There are many knowledge gaps regarding the pathophysiology and optimal management of placenta accreta. Given the relative infrequency of cases




at individual centers, multicenter and multinational cooperation and properly designed trials are needed to address these gaps. Another focus should be primary prevention by reducing the rate of unnecessary cesarean deliveries and exploring the influence of cesarean technique on the subsequent risk for accreta.

SUMMARY Maternal and fetal outcomes with placenta previa, vasa previa, and placenta accreta have improved considerably with antenatal diagnosis and modern management. Thus, it is important for clinicians to screen all pregnancies for these conditions at the time of their routine second-trimester ultrasonography. In addition, patients with risk factors (eg, multiple prior cesarean deliveries) should undergo targeted screening for placenta accreta. These conditions are all indications for iatrogenic preterm birth between 34 and 37 weeks of gestation. There continues to be substantial knowledge gaps regarding the optimal management of each of these conditions with regard to diagnosis, obstetric care, timing of delivery, and surgical management. Accordingly, most recommendations are based on expert opinion rather than quality evidence. Prospective clinical trials are needed to address these knowledge gaps to continue to improve outcomes in cases of previa, vasa previa, and accreta. REFERENCES

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Placenta previa.

Placenta previa.

The cervix as a natural tamponade in postpartum hemorrhage caused by placenta previa and placenta previa accreta: a prospective study.

Uterine tamponade-drain for hemorrhage secondary to placenta previa-accreta.

Migrating placenta previa.

Placenta Previa Unavoidable Hæmorrhage.

Fertility sparing surgery on placenta invasion anomalies and placenta previa.

Maternal and neonatal outcomes of placenta previa and placenta accreta: three years of experience with a two-consultant approach.

Effect of Placenta Previa on Preeclampsia.

Midtrimester placenta previa: normal or pathologic finding.

Is placenta previa a determinant of preeclampsia?

A new suture technique for cesarean delivery complicated by hemorrhage in cases of placenta previa accreta.

Cesarean hysterectomy for placenta previa accreta in dichorionic twin: a surgery that remains challenging.

Symbol "&" suture to control atonic postpartum hemorrhage with placenta previa accreta.

Is anterior placentation per se a risk for cesarean section-related morbidity in placenta previa?

Descendent migration in vasa previa.

Cervical tourniquet for anterior placenta previa: clarification and concern.

Prenatal diagnosis of vasa previa.

Does previa location matter? Surgical morbidity associated with location of a placenta previa.

Placenta previa in association with an incompetent cervix.

Natural history of placenta previa ascertained by diagnostic ultrasound.

Placenta previa percreta with bladder invasion presenting as incarcerated hernia.

Cigarette smoking during pregnancy in relation to placenta previa.

Cystotomy for placenta previa percreta with bladder invasion.