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ORIGINAL RESEARCH
Increasing Maternal Body Mass Index and the Accuracy of Sonographic Estimation of Fetal Weight Near Delivery Sara Kritzer, MD, Kristin Magner, MD, Carri R. Warshak, MD
Objectives—To evaluate whether an increasing body mass index (BMI) influences the accuracy of sonographic estimation of fetal weight. Methods—We performed a retrospective cohort study of singleton deliveries over a 2-year period in a single institution. Patients were included if they had a fetal weight estimation within 2 weeks of delivery. The Δ estimated fetal weight (EFW) was calculated by subtracting the sonographic EFW from the birth weight and compared among our study groups, which were based on the maternal BMI class. We also compared the absolute percentage error of estimation, rate of substantial error greater than 20%, rate of underestimation, and ability to predict fetal weight greater than 4000 g. Post hoc power analysis determined that our study group of 1200 patients allowed for an α of .05 and β of .90.
Received October 7, 2013, from the Department of Obstetrics and Gynecology, Division of MaternalFetal Medicine, University of Cincinnati College of Medicine, Cincinnati, Ohio USA (S.K., K.M., C.R.W.); and Department of Obstetrics and Gynecology, The Christ Hospital, Cincinnati, Ohio USA (S.K., K.M.). Revision requested November 19, 2013. Revised manuscript accepted for publication March 29, 2014. We thank the Center for Clinical and Translational Science and Training for assistance with development of the RedCap database. Address correspondence to Carri R. Warshak, MD, Department of Obstetrics and Gynecology, Division of Maternal-Fetal Medicine, University of Cincinnati College of Medicine, 231 Albert Sabin Way, Cincinnati, OH 45267-0526 USA. E-mail: [email protected]
Abbreviations
BMI, body mass index; EFW, estimated fetal weight doi:10.7863/ultra.33.12.2173
Results—We included 1177 women in our analysis. The median ΔEFW varied between study groups: 137, 202, 157, 200, and 189 g, respectively, in normal-weight, overweight, and obese classes 1, 2, and 3 (P = .01). The median percentage error of estimation between study groups varied between 5.0% in normal-weight women and 7.1% in class 2 obese women (P = .05). The rate of substantial error was similar between study groups and varied between 2.7% in class 1 obese women and 4.3% in normal-weight and class 2 obese women. Linear regression analysis showed a weak association between maternal BMI and ΔEFW (R2 = 0.005; r = 0.069). Conclusions—The absolute ΔEFW was lower in normal-weight women; however, the percentage error of the EFW was similar between women of varying BMI classifications, as was the rate of substantial error and the rate of underestimation of the EFW. Key Words—estimated fetal weight; macrosomia; obesity; obstetric ultrasound; sonography
T
he shifting demographic of the maternal body mass index (BMI) in pregnancy over the last decades is well documented. In a review from 1956, the rate of obesity in pregnancy was 3.6% (defined as weight >190 lb).1 In a cohort study from 1980, the rate of obesity was 10.1% (defined as weight >198 lb [90 kg]).2 A recent American College of Obstetricians and Gynecologists committee opinion, published in 2013, estimated that at least onethird of pregnant women are obese, and 8% are extremely obese.3 The increased incidence of overweight and obesity complicating pregnancy has immediate and far-reaching effects on the health of the mother and fetus, with increased risks of diabetes, preeclampsia, preterm delivery, thromboembolic disease, macrosomia, and stillbirth being well documented.4–15 Advances in our understanding
©2014 by the American Institute of Ultrasound in Medicine | J Ultrasound Med 2014; 33:2173–2179 | 0278-4297 | www.aium.org
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Kritzer et al—Increasing Maternal BMI and Estimation of Fetal Weight Near Delivery
of the fetal origins of disease have demonstrated associations between maternal obesity and childhood diseases such as obesity and metabolic syndrome.16–18 Maternal obesity has also been found to be associated with abnormal labor19–24 and fetal macrosomia.7,25,26 The prepregnancy BMI and gestational weight gain have been associated with an increased risk of surgical delivery.7,27–29 The detrimental impact maternal obesity has on the accuracy of sonography for detection of anomalies has been reported.30–32 However, analogous studies evaluating the influence of maternal obesity on accurate estimation of fetal weight are lacking. Historically, the error rate of sonography for estimation of fetal weight has been found to be as high as 25% in all-weight women.33 More recently, Goetzinger et al34 examined the sensitivity and specificity for extremes of abnormal fetal growth and found no association with the maternal BMI class. Whether an increasing maternal BMI affects the accuracy of the estimated fetal weight (EFW) is unreported, and given the added difficulty with imaging in this population, increased error is biologically feasible. Understanding whether maternal obesity decreases the accuracy of sonographic EFW is vital given known associations between an increasing maternal BMI and macrosomia, abnormal labor, and increasing surgical delivery rates. We performed this study to quantitate the impact, if any, an increasing maternal BMI has on the accuracy of sonographic EFW obtained within 2 weeks of delivery.
Materials and Methods We conducted a retrospective cohort study of all women who delivered a singleton live birth at the University of Cincinnati in 2008 and 2009. The study was approved by the Institutional Human Subjects Review Committee; the need for informed consent was waived given the retrospective nature of the study. We included all women with a known BMI who had obtained a sonographic EFW within 14 days of the date of delivery. We chose a 2-week period considering the known plateau in fetal growth in later pregnancy,35 although realizing there would be some fetal growth between the sonographic examination and delivery; by using a 2 week period later in pregnancy, this factor would be minimal. In addition, this time frame parallels clinical practice in that physicians are likely to schedule final growth scans near delivery but not necessarily simultaneous with delivery. Women were excluded if they delivered before 32 weeks or if they had a severe anomaly that could interfere with the ability to accurately estimate the fetal weight, such as an abdominal wall defect or skeletal
2174
dysplasia (Figure 1). We also excluded underweight women (BMI 4000 g, n (%) Medical complications, n (%) Pregestational diabetes Chronic hypertension Tobacco Obstetric complications, n (%) Gestational diabetes Preeclampsia Fetal growth restrictiona Induction Cesarean delivery
Normal (n = 161)
Overweight (n = 289)
Class 1 (n = 297)
Class 2 (n = 210)
Class 3 (n = 220)
23.8 ± 5.5 1 (0–2)
24.8 ± 6.0 1 (0–2)
26.6 ± 6.2 1 (0–3)
26.3 ± 5.6 1 (0–2)
26.6 ± 5.4 2 (1–3)
59 (14.8) 72 (12.0) 24 (15.9) 6 (31.6) 0 (0) 36 (34–38) 37 (35–39) 5±4 2687 ± 542 0 (0)
99 (24.8) 124 (20.7) 56 (37.1) 5 (26.3) 5 (50.0) 37 (36–39) 38 (37–39) 5±4 3004 ± 564 7 (2.4)
97 (24.3) 149 (24.9) 41 (27.2) 7 (2.4) 3 (30.0) 37 (36–39) 38 (37–40) 5±4 3163 ± 659 30 (10.1)
79 (19.8) 108 (18.1) 22 (14.6) 1 (5.3) 0 (0) 37 (36–39) 39 (38–40) 6±5 3224 ± 580 17 (8.1)
65 (16.3) 145 (24.2) 8 (5.3) 0 (0) 2 (20.0) 38 (36–38) 39 (37–39) 6±4 3268 ± 665 31 (14.1)
.19
ORIGINAL RESEARCH
Increasing Maternal Body Mass Index and the Accuracy of Sonographic Estimation of Fetal Weight Near Delivery Sara Kritzer, MD, Kristin Magner, MD, Carri R. Warshak, MD
Objectives—To evaluate whether an increasing body mass index (BMI) influences the accuracy of sonographic estimation of fetal weight. Methods—We performed a retrospective cohort study of singleton deliveries over a 2-year period in a single institution. Patients were included if they had a fetal weight estimation within 2 weeks of delivery. The Δ estimated fetal weight (EFW) was calculated by subtracting the sonographic EFW from the birth weight and compared among our study groups, which were based on the maternal BMI class. We also compared the absolute percentage error of estimation, rate of substantial error greater than 20%, rate of underestimation, and ability to predict fetal weight greater than 4000 g. Post hoc power analysis determined that our study group of 1200 patients allowed for an α of .05 and β of .90.
Received October 7, 2013, from the Department of Obstetrics and Gynecology, Division of MaternalFetal Medicine, University of Cincinnati College of Medicine, Cincinnati, Ohio USA (S.K., K.M., C.R.W.); and Department of Obstetrics and Gynecology, The Christ Hospital, Cincinnati, Ohio USA (S.K., K.M.). Revision requested November 19, 2013. Revised manuscript accepted for publication March 29, 2014. We thank the Center for Clinical and Translational Science and Training for assistance with development of the RedCap database. Address correspondence to Carri R. Warshak, MD, Department of Obstetrics and Gynecology, Division of Maternal-Fetal Medicine, University of Cincinnati College of Medicine, 231 Albert Sabin Way, Cincinnati, OH 45267-0526 USA. E-mail: [email protected]
Abbreviations
BMI, body mass index; EFW, estimated fetal weight doi:10.7863/ultra.33.12.2173
Results—We included 1177 women in our analysis. The median ΔEFW varied between study groups: 137, 202, 157, 200, and 189 g, respectively, in normal-weight, overweight, and obese classes 1, 2, and 3 (P = .01). The median percentage error of estimation between study groups varied between 5.0% in normal-weight women and 7.1% in class 2 obese women (P = .05). The rate of substantial error was similar between study groups and varied between 2.7% in class 1 obese women and 4.3% in normal-weight and class 2 obese women. Linear regression analysis showed a weak association between maternal BMI and ΔEFW (R2 = 0.005; r = 0.069). Conclusions—The absolute ΔEFW was lower in normal-weight women; however, the percentage error of the EFW was similar between women of varying BMI classifications, as was the rate of substantial error and the rate of underestimation of the EFW. Key Words—estimated fetal weight; macrosomia; obesity; obstetric ultrasound; sonography
T
he shifting demographic of the maternal body mass index (BMI) in pregnancy over the last decades is well documented. In a review from 1956, the rate of obesity in pregnancy was 3.6% (defined as weight >190 lb).1 In a cohort study from 1980, the rate of obesity was 10.1% (defined as weight >198 lb [90 kg]).2 A recent American College of Obstetricians and Gynecologists committee opinion, published in 2013, estimated that at least onethird of pregnant women are obese, and 8% are extremely obese.3 The increased incidence of overweight and obesity complicating pregnancy has immediate and far-reaching effects on the health of the mother and fetus, with increased risks of diabetes, preeclampsia, preterm delivery, thromboembolic disease, macrosomia, and stillbirth being well documented.4–15 Advances in our understanding
©2014 by the American Institute of Ultrasound in Medicine | J Ultrasound Med 2014; 33:2173–2179 | 0278-4297 | www.aium.org
3312jum_online_Layout 1 11/24/14 10:19 AM Page 2174
Kritzer et al—Increasing Maternal BMI and Estimation of Fetal Weight Near Delivery
of the fetal origins of disease have demonstrated associations between maternal obesity and childhood diseases such as obesity and metabolic syndrome.16–18 Maternal obesity has also been found to be associated with abnormal labor19–24 and fetal macrosomia.7,25,26 The prepregnancy BMI and gestational weight gain have been associated with an increased risk of surgical delivery.7,27–29 The detrimental impact maternal obesity has on the accuracy of sonography for detection of anomalies has been reported.30–32 However, analogous studies evaluating the influence of maternal obesity on accurate estimation of fetal weight are lacking. Historically, the error rate of sonography for estimation of fetal weight has been found to be as high as 25% in all-weight women.33 More recently, Goetzinger et al34 examined the sensitivity and specificity for extremes of abnormal fetal growth and found no association with the maternal BMI class. Whether an increasing maternal BMI affects the accuracy of the estimated fetal weight (EFW) is unreported, and given the added difficulty with imaging in this population, increased error is biologically feasible. Understanding whether maternal obesity decreases the accuracy of sonographic EFW is vital given known associations between an increasing maternal BMI and macrosomia, abnormal labor, and increasing surgical delivery rates. We performed this study to quantitate the impact, if any, an increasing maternal BMI has on the accuracy of sonographic EFW obtained within 2 weeks of delivery.
Materials and Methods We conducted a retrospective cohort study of all women who delivered a singleton live birth at the University of Cincinnati in 2008 and 2009. The study was approved by the Institutional Human Subjects Review Committee; the need for informed consent was waived given the retrospective nature of the study. We included all women with a known BMI who had obtained a sonographic EFW within 14 days of the date of delivery. We chose a 2-week period considering the known plateau in fetal growth in later pregnancy,35 although realizing there would be some fetal growth between the sonographic examination and delivery; by using a 2 week period later in pregnancy, this factor would be minimal. In addition, this time frame parallels clinical practice in that physicians are likely to schedule final growth scans near delivery but not necessarily simultaneous with delivery. Women were excluded if they delivered before 32 weeks or if they had a severe anomaly that could interfere with the ability to accurately estimate the fetal weight, such as an abdominal wall defect or skeletal
2174
dysplasia (Figure 1). We also excluded underweight women (BMI 4000 g, n (%) Medical complications, n (%) Pregestational diabetes Chronic hypertension Tobacco Obstetric complications, n (%) Gestational diabetes Preeclampsia Fetal growth restrictiona Induction Cesarean delivery
Normal (n = 161)
Overweight (n = 289)
Class 1 (n = 297)
Class 2 (n = 210)
Class 3 (n = 220)
23.8 ± 5.5 1 (0–2)
24.8 ± 6.0 1 (0–2)
26.6 ± 6.2 1 (0–3)
26.3 ± 5.6 1 (0–2)
26.6 ± 5.4 2 (1–3)
59 (14.8) 72 (12.0) 24 (15.9) 6 (31.6) 0 (0) 36 (34–38) 37 (35–39) 5±4 2687 ± 542 0 (0)
99 (24.8) 124 (20.7) 56 (37.1) 5 (26.3) 5 (50.0) 37 (36–39) 38 (37–39) 5±4 3004 ± 564 7 (2.4)
97 (24.3) 149 (24.9) 41 (27.2) 7 (2.4) 3 (30.0) 37 (36–39) 38 (37–40) 5±4 3163 ± 659 30 (10.1)
79 (19.8) 108 (18.1) 22 (14.6) 1 (5.3) 0 (0) 37 (36–39) 39 (38–40) 6±5 3224 ± 580 17 (8.1)
65 (16.3) 145 (24.2) 8 (5.3) 0 (0) 2 (20.0) 38 (36–38) 39 (37–39) 6±4 3268 ± 665 31 (14.1)
.19
