Early pregnancy assessment with transvaginal ultrasound scanning Salim Daya,* MB, MSc; Sheila Woods,t MD; Susan Ward,* RN; Ray Lappalainen,t RDMS; Chris Caco,t MD Objective: To establish normal parameters in early pregnancy through transvaginal ultrasonography so that gestational age can be determined and to correlate the sonographic findings with serum human chorionic gonadotropin (hCG) levels calibrated against the first international reference preparation standard. Setting:'Infertility clinic. Patients: Thirty-five women with normal intrauterine pregnancy. Interventions: 'Serial measurement of the serum hCG level and the diameter of the gestational sac through transvaginal ultrasonography. Main results: The gestational sac could not be visualized when the hCG level was less than 1 100 IU/L. The average growth rate of the sac was 0.9 mm/d. The threshold values for sac diameter, serum hCG level and gestational age below which the yolk sac was not visible were 3.7 mm, 1900 IU/L and 36 days respectively; the corresponding values above which the yolk sac was always visible were 6.7 mm, 5800 IU/L and' 40 days. The threshold values below which cardiac activity was not visible were 8.3 mm, 9200 IU/L and 41 days respectively, and the corresponding values above which cardiac activity was always visible were 14.0 mm, 24 000 IU/L and 46 days. The mean gestational ages and the 95% confidence and prediction intervals were tabulated so that measurement of the gestational sac diameter could be used to estimate gestational age early in normal pregnancy. Conclusions: Transvaginal ultrasonography enables detection of an intrauterine sac and reliable estimation 'of gestational age on the basis of sac dimensions before an embryo can be seen.
Objectif: ktablir, par ultrasonographie transvaginale, les mesures normales au debut de la grossesse afin de pouvoir determiner l'age gestationnel et etablir un lien entre les constatations sonographiques et les niveaux seriques de gonadotrophine chorionique humaine (GCH) etalonnes en fonction du premier etalon international de preparation de reference. Coptexte: Clinique de traitement de l'infertilite. Patients: Trente-cinq femmes en etat de grossesse intrauterine normale. Interventions: Ser,ie de mesures du niveau serique de GCH et, par ultrasonographie transvaginale, du diametre du sac gestationnel. Resultats principaux : On ne pouvait visualiser le sac gestationnel lorsque le niveau de GCH etait inferieur a 1 100 UI/L. Le taux de croissance moyen du sac etait de 0,9 mm/j. Les valeurs minimales du diametre du sac, du niveau serique GCH 'et de l'age gestationnel au-dessous desquelles la vesicule vitelline etait invisible etaient respectivement de 3,7 mm, 1 900 UI/L et 36 jours. Les valeurs correspondantes au-dessus desquelles la vesicule vitelline etait toujours visible etaient de 6,7 mm, 5 800 UI/L et 40 jours. Les valeurs minimales au-dessous desquelles l'activite cardiaque n'etait pas visible etaient respectivement de 8,3 mm, 9 200 UI/L et 41 jours, et les valeurs correspondantes From the departments of *Obstetrics and Gynaecology and tRadiology, McMaster University, Hamilton, Ont. Dr. Daya is a career scientist of the Ontario Ministry of Health.
Reprint requests to: Dr. Salim Daya, Department of Obstetrics and Gynaecology, McMaster University, 1200 Main St. W, Hamilton, ON L8N 3Z5 CAN MED ASSOC J 1991; 144 (4)
441
au-dessus desquelles l'activite cardiaque etait toujours visible etaient de 14,0 mm, 24 000 UI/L et 46 jours. On a etabli le tableau des ages gestationnels moyens et des intervalles de prediction et de confiance a 95 % afin de pouvoir utiliser le diametre du sac gestationnel pour evaluer l'age gestationnel t6t au cours d'une grossesse normale. Conclusion: L'ultrasonographie transvaginale permet de detecter la presence d'un sac intrauterin et d'effectuer une estimation fiable de l'age gestationnel en fonction des dimensions du sac avant que l'embryon ne devienne visible.
U ltrasonography is an important tool in the those determined with kits calibrated against the 2nd assessment of pregnancy, and its use in IS.' Thus, it is not meaningful to use a single clinical diagnosis has increased dramatically conversion factor unless the two standards are used with improvements in imaging technology. The re- in the same assay - a technique that is not practicent development of transvaginal ultrasonography cal.8 Consequently, when relying on hCG quantitahas resulted in better images because of the place- tion for decision-making one must be aware of the ment of higher frequency transducers closer to the standard used for calibration in the laboratory where pelvic organs. This technique has enabled the detec- the test was done. We performed this study (a) to evaluate the use tion of intrauterine pregnancy earlier than with the transabdominal approach and has become particu- of transvaginal ultrasonography in early pregnancy larly useful in the evaluation of ectopic and other in order to establish normal parameters for gestational age assessment by measuring gestational sac abnormal pregnancies. ' The use of radioimmunoassay for human chori- diameters and (b) to correlate the sonographic aponic gonadotropin (hCG) has also allowed early pearance of early embryonic structures with serum detection of pregnancy. Several attempts have been hCG levels determined with kits calibrated against made to correlate the ultrasonographic appearances the 1st IRP standard. Similar studies have reof early intrauterine pregnancy with simultaneous cently been reported in which the serum hCG quantitative measurements of hCG so that normal levels were determined with kits calibrated against pregnancies can be distinguished from abnormal the 2nd IS.6'9 ones.2'3 These correlations have led to the definition of an hCG discriminatory zone, above which an Methods intrauterine gestational sac could be detected. With Thirty-five women who conceived as a result of the use of transabdominal scanning this zone was found to represent serum hCG levels of 6000 to treatment in the infertility clinic at Chedoke6500 1U/L.2 The improved sensitivity of transvaginal McMaster Hospitals, Hamilton, Ont., participated in ultrasonography necessitates re-evaluation of the the study. All had singleton intrauterine pregnancies that progressed beyond 20 weeks' gestation. The zone. Since hCG has not been completely character- pregnancies were confirmed through serum hCG ized chemically, all commercially available hCG testing, and their progress was monitored by serial radioimmunoassay kits must be calibrated against an hCG measurements performed every 2 to 7 days (3 agreed-upon standard. In 1975 the World Health to 4 days on average) until about 60 days' gestation Organization established the first international refer- and by transvaginal ultrasound scanning every 1 to 2 ence preparation (1st IRP) of hCG for immunoassay weeks until 8 weeks' gestation. Quantitation of hCG and recommended that it be used to calibrate all was performed by means of radioimmunoassay dicommercial hCG kits.4 Unfortunately not all kit rected against the ,B subunit of the molecule and manufacturers adhere to this recommendation: calibrated against the 1st IRP standard.'0 Gestational age was determined by each of two many still calibrate their kits against the older, relatively impure and heterogeneous second interna- methods. Menstrual gestational age was calculated tional standard (2nd IS) for hCG bioassay, estab- from the first day of the last menstrual period. True lished in 1964.5 This has significant implications for (corrected) gestational age was calculated with the physicians relying on quantitative hCG determina- use of the normal curve for hCG production in early tions to aid in clinical diagnosis. To compare the pregnancy.'0 The natural logarithm of the hCG level 2nd IS with the purer 1st IRP standard a conversion was plotted against the gestational age, and the factor has been suggested: 1 IU/L of the 2nd IS resulting curve was compared with the normal curve. fl-hCG = 2.2 IU/L of the 1st IRP.6 However, in an The assumption underlying the use of the normal analysis of 10 different commercial kits Rasor, curve was that ovulation occurred on day 14 of a Farber and Braunstein7 found that the serum hCG 28-day menstrual cycle. The validity of this assumpvalues determined with kits calibrated against the 1 st tion has been confirmed in patients achieving pregIRP standard were 1.75 to 3.5 times higher than nancy through in-vitro fertilization, in which oocyte 442
CAN MED ASSOC J 1991; 144 (4)
retrieval was retrospectively assigned as being on day 14 of gestation.""I2 Thus, the normal curve represents hCG production during the early stages of an ideal gestational period of 280 days. If the patient's hCG curve fell to the right of the normal curve, then ovulation was assumed to have occurred after day 14, and the average number of days between these two curves was subtracted from the menstrual gestational age. Similarly, if the curve fell to the left, then ovulation was assumed to have occurred before day 14, and the relevant number of days was added to the menstrual gestational age. Using pregnancy data from in-vitro fertilization treatment we found that the mean difference between the actual gestational age and that expected from the normal curve was 0.44 (standard deviation [SD] 0.43) days, 84% of the data points falling within day of the curve.'2 These observations indicate that the normal curve is very precise for calculating gestational age. All ultrasound measurements were performed transvaginally with the use of an Aloka SSD-650 machine fitted with a 5-MHz convex sector probe with a 600 scanning angle, 0.5-mm axial resolution and 1.7-mm lateral resolution (Omnium Medical Devices of Canada Inc., Richmond Hill, Ont.). The mean diameter of the gestational sac was calculated on the basis of measurements of the inner sac wall in three planes at right angles to each other. The sac was thoroughly scanned to identify a yolk sac and a fetal pole; when the latter was identified much effort was taken to determine whether cardiac activity was
Results Data from 113 ultrasound measurements were evaluated along with the corresponding serum hCG levels. In five cases the gestational sac was not visualized when the hCG level was less than 1100 IU/L; the mean hCG level was 650 (SD 356) IU/L (extremes 214 and 1091 IU/L). Above this threshold level the gestational sac was always visualized. Table 1 shows the cutoff points for mean sac diameter, true gestational age and serum hCG level below which a yolk sac and cardiac activity were not identified and above which they were always present. We found a linear relation between true gestational age and mean sac diameter (r = 0.95, p < 0.0001) and between menstrual gestational age and mean sac diameter (r = 0.77, p < 0.0001) (Figs. 1 and 2). Since the correlation coefficient was much better with true gestational age than with menstrual gestational age all further inferences were made with the use of the true gestational age regression model. 70
-
i
=60
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00 30
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present. 20-
Statistical analysis
10
The relation between gestational age and mean diameter was explored through regression analysis. Student's t-test was used to determine whether the slopes of the regression lines were significantly different from zero. We identified cutoff points for mean sac diameter, true gestational age and serum hCG levels below which embryonic characteristics were not visible on ultrasonography and above which they were always visible. sac
-
10
0
5
15
10
20
Mean sac diameter,
25
30
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Fig. 1: Relation between mean diameter of gestational sac and true gestational age. Equation for regression line (solid line) was as follows: gestational age (in days) = 0.882(mean sac diameter [in millimetres]) + 33.117. Dotted lines represent upper and lower limits of 95% prediction interval (PI).
Table 1: Discriminatory measurements in early pregnancy with respect to visualization of embryonic characteristics by transvaginal ultrasonography*
Characteristic Yolk sac Cardiac activity
Mean diameter of gestational sac, mm N P 3.7 6.7 8.3 14.0
True gestational age, d P N 40 36 41
46
Serum hCG
level,t IU/L
N 1 900 9 200
P 5 800 24 000
*For each parameter the discriminatory measurements are values below which the characteristics were not visible (N) and above which they were always present (P). thCG = human chorionic gonadotropin. CAN MED ASSOC J 1991; 144 (4)
443
The slope of the regression line was significantly lack of fit of the model to the data (p = 0.207). In early pregnancy the rate of growth of the different from zero (t = 38.52 with 111 degrees of freedom, p < 0.0001), and there was no significant gestational sac was about 0.9 mm/d. Using the regression equation gestational age (in days) = 80 0.882(mean sac diameter [in millimetres]) + 33.117 calculated the gestational age for various mean we 70sac diameters (Table 2). The 95% confidence interval r 0.77 (CI) for the mean gestational age and the 95% prediction interval (PI) for the observed gestational C0 50 age were also calculated (Table 2, Fig. 3). The reliability of the data in Table 2 was determined in 10 women with a singleton intrauter30-...ine pregnancy resulting from in-vitro fertilization. The mean gestational sac diameter was determined - 20 through transvaginal ultrasonography, and the gestaU) 0 10 tional age expected from Table 2 was compared with the actual age. The variability observed between and actual gestational age was 1 day (mean expected 35 20 25 30 15 5 10 0 0.2 0.6] days) (Table 3). [SD Mean sac diameter, mm -
Discussion
Fig. 2: Relation between mean diameter of gestati and menstrual gestational age. Equation for regres: (solid line) was as follows: gestational age (in 0.821(mean sac diameter [in millimetresi) + 35.081 lines represent upper and lower limits of 95% PI. Mear diamete-
'TaOIe
Estimation of gestational age on the basis of knowledge of the last menstrual period is well known to be error-prone. Our results further support this --)rresporlding estimate,s
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observation, since a greater degree of variability was observed when menstrual data were used than when hCG measurements were used. Also, the use of the normal hCG curve to correct for uncertainties in the date of ovulation has been shown to predict gestational age accurately."'12 Our findings confirm that an intrauterine gestational sac can be detected very early with the use of transvaginal ultrasonography and that the hCG threshold level is much lower than previously reported with the use of transabdominal ultrasonography. Because of the small number of cases in which no gestational sac was visualized a practical hCG threshold level could not be determined reliably from our data. Nevertheless, a gestational sac was not observed when the hCG level was less than 1 100
IU/L. This was further reinforced in our subsequent study in which we correlated hCG levels with gestational sac diameter.'3 By extrapolating the curve and calculating the upper 95% PI at a mean sac diameter of 0 mm we found that the hCG level obtained was 1079 IU/L. Similar hCG levels have been reported by others.'4"'5 One group of investigators identified a gestational sac at much lower hCG levels;'6 however, since they did not state which hCG standard was used it is tempting to speculate that it was the 2nd IS.
The fact that during early pregnancy the rate of growth of the gestational sac is linear enables the true gestational age to be calculated. Using the regression equation we tabulated the expected gestational ages so that the mean sac diameter could be converted to the corresponding gestational age. This table was shown to be very reliable and precise in the 70 assessment of pregnancies conceived through invitro fertilization. Therefore, it can be used clinically 60 r 0.95 to determine gestational age. One gold standard used for ultrasonic assessment of gestational age requires la precise measurement of the crown-rump length. This measurement is usually not possible until later cn ~40in the first trimester, when the fetus can be clearly visualized. However, with the use of transvaginal *_ 30 ultrasonography the gestational age can now be accurately determined through the measurement of 0)20the sac diameter. Furthermore, one can monitor a 10 pregnancy by comparing the size of the gestational sac in two ultrasound examinations performed several days apart. This method may provide useful 0 5 10 15 20 25 30 35 prognostic information to women with a history of Mean sac diameter, mm recurrent abortion who are carefully monitored by Fig. 3: Mean diameter of gestational sac and corresponding ultrasonography in early pregnancy and to women estimates of gestational age. Solid line represents mean who are spotting in early pregnancy, especially if gestational age, dotted lines represent upper and lower appropriate growth in sac size is observed. Early identification of intrauterine pregnancy limits of 95% confidence interval, and broken lines represent upper and lower limits of 95% PI. could also improve clinical management of ectopic -
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Table 3: Actual and expected gestational ages in 10 pregnancies conceived through in-vitro fertilization Patient no. 1 2 3 4 5 6 7 8 9 10
Gestational
Mean sac diameter, mm 7 8 17 7 15 9 25 16
Actual 39 40 47 39 46 40 55 46
11 12
44
44
age,* d
Expected 39 40 48 39 46 41 55 47 43 44
Difference 0 0 +1 0 0 +1
0 +1 -1
0
*Actual gestational age was calculated by retrospectively assigning the day of oocyte retrieval as day 14 of gestation. Expected gestational age was obtained from Table 2 after the sac diameter was determined through transvaginal ultrasound scanning. CAN MED ASSOC J 1991; 144 (4)
445
pregnancy. With the use of radioimmunoassay for hCG, pregnancy can be confirmed within 2 weeks after ovulation."7,8 Several investigators have described discriminatory zones using hCG levels to aid in the ultrasonic diagnosis of ectopic pregnancy.2'3 The presence of a yolk sac within the gestational sac in the uterus provides reassuring evidence that there is no ectopic pregnancy. Such a structure could not be visualized with transvaginal ultrasonography when the hCG level was less than 1900 IU/L. In addition, direct visualization of the ectopic pregnancy in the adnexa is now sometimes possible with the use of transvaginal ultrasonography.19,20 Thus, our diagnostic ability has been markedly improved with the advent of transvaginal ultrasound scanning.
References 1. Nyberg DA, Mack LA, Jeffrey RB et al: Endovaginal sonographic evaluation of ectopic pregnancy: a prospective study. AJR 1987; 149: 1181-1186 2. Kadar N, DeVore G, Romero R: Discriminatory hCG zone: its use in the sonographic evaluation for ectopic pregnancy. Obstet Gynecol 1981; 58: 156-161 3. Batzer FR, Weiner S, Corson SL et al: Landmarks during the first forty-two days of gestation demonstrated by the betasubunit of human chorionic gonadotropin and ultrasound. Am J Obstet Gynecol 1983; 146: 973-979 4. Canfield RE, Ross GT: A new reference preparation of human chorionic gonadotropin and its subunits. Bull WHO 1976; 54: 463-470 5. Bangham DR, Grab B: The Second International Standard for chorionic gonadotrophin. Bull WHO 1964; 31: 111-125 6. Bernaschek G, Rudelstorfer R, Csaicsich P: Vaginal sonography versus serum human chorionic gonadotropin in early detection of pregnancy. Am J Obstet Gynecol 1988; 158: 608612 7. Rasor JL, Farber S, Braunstein GD: An evaluation of 10 kits for determination of human choriogonadotropin in serum.
Clin Chem 1983; 29: 1828-1831 8. Kadar N, Romero R: Discriminatory human chorionic gonadotropin zone "demilitarized." Am J Obstet Gynecol 1989; 160: 1255-1256 9. Nyberg DA, Mack LA, Laing FC et al: Early pregnancy complications: endovaginal sonographic findings correlated with human chorionic gonadotropin levels. Radiology 1988; 167: 619-622 10. Daya S: Human chorionic gonadotropin increase in normal early pregnancy. Am J Obstet Gynecol 1987; 156: 286-290 11. Daya S, Woods S, Ward S et al: Early pregnancy assessment with transvaginal ultrasound scanning [abstr 58]. Presented at the 45th Annual Meeting of the Society of Obstetricians and Gynaecologists of Canada, Quebec, June 20-24, 1989 12. Daya S: Determination of gestational age by measuring human chorionic gonadotropin levels. In Boutaleb Y, Gzouli A (eds): The Study of Ovulation and Early Pregnancy (Recent Developments in Fertility and Sterility ser, vol 2), Parthenon, Carnforth, England, 1990: 159-162 13. Daya S, Woods S, Ward S et al: Transvaginal ultrasound scanning and correlation with human chorionic gonadotropin levels. J Clin Ultrasound (in press) 14. Goldstein SR, Snyder JR, Watson C et al: Very early pregnancy detection with endovaginal ultrasound. Obstet Gynecol 1988; 72: 200-204 15. Fossum GT, Davajan V, Kletzky OA: Early detection of pregnancy with transvaginal ultrasound. Fertil Steril 1988; 49: 788-791 16. Timor-Tritsch IE, Farine D, Rosen MG: A close look at early embryonic development with the high-frequency transvaginal transducer. Am J Obstet Gynecol 1988; 159: 676-681 17. Kosasa T, Levesque L, Taymor M et al: Measurement of early chorionic activity with a radioimmune assay specific for human chorionic gonadotropin following spontaneous and induced ovulation. Fertil Steril 1974; 25: 211-216 18. Braunstein GD, Grodin JM, Vaitukaitis J et al: Secretory rates of human chorionic gonadotropin by normal trophoblast. Am J Obstet Gynecol 1973; 115: 447-450 19. Shapiro BS, Cullen M, Taylor KJW et al: Transvaginal ultrasonography for the diagnosis of ectopic pregnancy. Fertil Steril 1988; 50: 425-429 20. Rempen A: Vaginal sonography in ectopic pregnancy. A prospective evaluation. J Ultrasound Med 1988; 7: 381-387
Keeping in fashion I haven't the slightest idea where fashions in pathology are born.... Possibly some of my older readers dimly recollect the days when modish scientists declared that the only dependable method of relieving a toothache was a clean, conclusive appendectomy. Then the whistle blew for the quarter, the two teams changed goals, and it developed that ifyou had a pain in your side it was high time your teeth came out. Late in the second half, tonsils got into the game and broke awayfor a long run; two plays later, colitis recovered a fumble and kicked out of danger, and the issue has been terribly confused ever since. Only one point is clear: it's better to be dead, or even perfectly well, than to suffer from the wrong affliction. The man who owns up to arthritis in a beriberi year is as lonely as a woman in a last month's dress.
Ogden Nash (1902-1971)
446
CAN MED ASSOC J 1991; 144 (4)
Objectif: ktablir, par ultrasonographie transvaginale, les mesures normales au debut de la grossesse afin de pouvoir determiner l'age gestationnel et etablir un lien entre les constatations sonographiques et les niveaux seriques de gonadotrophine chorionique humaine (GCH) etalonnes en fonction du premier etalon international de preparation de reference. Coptexte: Clinique de traitement de l'infertilite. Patients: Trente-cinq femmes en etat de grossesse intrauterine normale. Interventions: Ser,ie de mesures du niveau serique de GCH et, par ultrasonographie transvaginale, du diametre du sac gestationnel. Resultats principaux : On ne pouvait visualiser le sac gestationnel lorsque le niveau de GCH etait inferieur a 1 100 UI/L. Le taux de croissance moyen du sac etait de 0,9 mm/j. Les valeurs minimales du diametre du sac, du niveau serique GCH 'et de l'age gestationnel au-dessous desquelles la vesicule vitelline etait invisible etaient respectivement de 3,7 mm, 1 900 UI/L et 36 jours. Les valeurs correspondantes au-dessus desquelles la vesicule vitelline etait toujours visible etaient de 6,7 mm, 5 800 UI/L et 40 jours. Les valeurs minimales au-dessous desquelles l'activite cardiaque n'etait pas visible etaient respectivement de 8,3 mm, 9 200 UI/L et 41 jours, et les valeurs correspondantes From the departments of *Obstetrics and Gynaecology and tRadiology, McMaster University, Hamilton, Ont. Dr. Daya is a career scientist of the Ontario Ministry of Health.
Reprint requests to: Dr. Salim Daya, Department of Obstetrics and Gynaecology, McMaster University, 1200 Main St. W, Hamilton, ON L8N 3Z5 CAN MED ASSOC J 1991; 144 (4)
441
au-dessus desquelles l'activite cardiaque etait toujours visible etaient de 14,0 mm, 24 000 UI/L et 46 jours. On a etabli le tableau des ages gestationnels moyens et des intervalles de prediction et de confiance a 95 % afin de pouvoir utiliser le diametre du sac gestationnel pour evaluer l'age gestationnel t6t au cours d'une grossesse normale. Conclusion: L'ultrasonographie transvaginale permet de detecter la presence d'un sac intrauterin et d'effectuer une estimation fiable de l'age gestationnel en fonction des dimensions du sac avant que l'embryon ne devienne visible.
U ltrasonography is an important tool in the those determined with kits calibrated against the 2nd assessment of pregnancy, and its use in IS.' Thus, it is not meaningful to use a single clinical diagnosis has increased dramatically conversion factor unless the two standards are used with improvements in imaging technology. The re- in the same assay - a technique that is not practicent development of transvaginal ultrasonography cal.8 Consequently, when relying on hCG quantitahas resulted in better images because of the place- tion for decision-making one must be aware of the ment of higher frequency transducers closer to the standard used for calibration in the laboratory where pelvic organs. This technique has enabled the detec- the test was done. We performed this study (a) to evaluate the use tion of intrauterine pregnancy earlier than with the transabdominal approach and has become particu- of transvaginal ultrasonography in early pregnancy larly useful in the evaluation of ectopic and other in order to establish normal parameters for gestational age assessment by measuring gestational sac abnormal pregnancies. ' The use of radioimmunoassay for human chori- diameters and (b) to correlate the sonographic aponic gonadotropin (hCG) has also allowed early pearance of early embryonic structures with serum detection of pregnancy. Several attempts have been hCG levels determined with kits calibrated against made to correlate the ultrasonographic appearances the 1st IRP standard. Similar studies have reof early intrauterine pregnancy with simultaneous cently been reported in which the serum hCG quantitative measurements of hCG so that normal levels were determined with kits calibrated against pregnancies can be distinguished from abnormal the 2nd IS.6'9 ones.2'3 These correlations have led to the definition of an hCG discriminatory zone, above which an Methods intrauterine gestational sac could be detected. With Thirty-five women who conceived as a result of the use of transabdominal scanning this zone was found to represent serum hCG levels of 6000 to treatment in the infertility clinic at Chedoke6500 1U/L.2 The improved sensitivity of transvaginal McMaster Hospitals, Hamilton, Ont., participated in ultrasonography necessitates re-evaluation of the the study. All had singleton intrauterine pregnancies that progressed beyond 20 weeks' gestation. The zone. Since hCG has not been completely character- pregnancies were confirmed through serum hCG ized chemically, all commercially available hCG testing, and their progress was monitored by serial radioimmunoassay kits must be calibrated against an hCG measurements performed every 2 to 7 days (3 agreed-upon standard. In 1975 the World Health to 4 days on average) until about 60 days' gestation Organization established the first international refer- and by transvaginal ultrasound scanning every 1 to 2 ence preparation (1st IRP) of hCG for immunoassay weeks until 8 weeks' gestation. Quantitation of hCG and recommended that it be used to calibrate all was performed by means of radioimmunoassay dicommercial hCG kits.4 Unfortunately not all kit rected against the ,B subunit of the molecule and manufacturers adhere to this recommendation: calibrated against the 1st IRP standard.'0 Gestational age was determined by each of two many still calibrate their kits against the older, relatively impure and heterogeneous second interna- methods. Menstrual gestational age was calculated tional standard (2nd IS) for hCG bioassay, estab- from the first day of the last menstrual period. True lished in 1964.5 This has significant implications for (corrected) gestational age was calculated with the physicians relying on quantitative hCG determina- use of the normal curve for hCG production in early tions to aid in clinical diagnosis. To compare the pregnancy.'0 The natural logarithm of the hCG level 2nd IS with the purer 1st IRP standard a conversion was plotted against the gestational age, and the factor has been suggested: 1 IU/L of the 2nd IS resulting curve was compared with the normal curve. fl-hCG = 2.2 IU/L of the 1st IRP.6 However, in an The assumption underlying the use of the normal analysis of 10 different commercial kits Rasor, curve was that ovulation occurred on day 14 of a Farber and Braunstein7 found that the serum hCG 28-day menstrual cycle. The validity of this assumpvalues determined with kits calibrated against the 1 st tion has been confirmed in patients achieving pregIRP standard were 1.75 to 3.5 times higher than nancy through in-vitro fertilization, in which oocyte 442
CAN MED ASSOC J 1991; 144 (4)
retrieval was retrospectively assigned as being on day 14 of gestation.""I2 Thus, the normal curve represents hCG production during the early stages of an ideal gestational period of 280 days. If the patient's hCG curve fell to the right of the normal curve, then ovulation was assumed to have occurred after day 14, and the average number of days between these two curves was subtracted from the menstrual gestational age. Similarly, if the curve fell to the left, then ovulation was assumed to have occurred before day 14, and the relevant number of days was added to the menstrual gestational age. Using pregnancy data from in-vitro fertilization treatment we found that the mean difference between the actual gestational age and that expected from the normal curve was 0.44 (standard deviation [SD] 0.43) days, 84% of the data points falling within day of the curve.'2 These observations indicate that the normal curve is very precise for calculating gestational age. All ultrasound measurements were performed transvaginally with the use of an Aloka SSD-650 machine fitted with a 5-MHz convex sector probe with a 600 scanning angle, 0.5-mm axial resolution and 1.7-mm lateral resolution (Omnium Medical Devices of Canada Inc., Richmond Hill, Ont.). The mean diameter of the gestational sac was calculated on the basis of measurements of the inner sac wall in three planes at right angles to each other. The sac was thoroughly scanned to identify a yolk sac and a fetal pole; when the latter was identified much effort was taken to determine whether cardiac activity was
Results Data from 113 ultrasound measurements were evaluated along with the corresponding serum hCG levels. In five cases the gestational sac was not visualized when the hCG level was less than 1100 IU/L; the mean hCG level was 650 (SD 356) IU/L (extremes 214 and 1091 IU/L). Above this threshold level the gestational sac was always visualized. Table 1 shows the cutoff points for mean sac diameter, true gestational age and serum hCG level below which a yolk sac and cardiac activity were not identified and above which they were always present. We found a linear relation between true gestational age and mean sac diameter (r = 0.95, p < 0.0001) and between menstrual gestational age and mean sac diameter (r = 0.77, p < 0.0001) (Figs. 1 and 2). Since the correlation coefficient was much better with true gestational age than with menstrual gestational age all further inferences were made with the use of the true gestational age regression model. 70
-
i
=60
r0.95
00 30
-
present. 20-
Statistical analysis
10
The relation between gestational age and mean diameter was explored through regression analysis. Student's t-test was used to determine whether the slopes of the regression lines were significantly different from zero. We identified cutoff points for mean sac diameter, true gestational age and serum hCG levels below which embryonic characteristics were not visible on ultrasonography and above which they were always visible. sac
-
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Fig. 1: Relation between mean diameter of gestational sac and true gestational age. Equation for regression line (solid line) was as follows: gestational age (in days) = 0.882(mean sac diameter [in millimetres]) + 33.117. Dotted lines represent upper and lower limits of 95% prediction interval (PI).
Table 1: Discriminatory measurements in early pregnancy with respect to visualization of embryonic characteristics by transvaginal ultrasonography*
Characteristic Yolk sac Cardiac activity
Mean diameter of gestational sac, mm N P 3.7 6.7 8.3 14.0
True gestational age, d P N 40 36 41
46
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level,t IU/L
N 1 900 9 200
P 5 800 24 000
*For each parameter the discriminatory measurements are values below which the characteristics were not visible (N) and above which they were always present (P). thCG = human chorionic gonadotropin. CAN MED ASSOC J 1991; 144 (4)
443
The slope of the regression line was significantly lack of fit of the model to the data (p = 0.207). In early pregnancy the rate of growth of the different from zero (t = 38.52 with 111 degrees of freedom, p < 0.0001), and there was no significant gestational sac was about 0.9 mm/d. Using the regression equation gestational age (in days) = 80 0.882(mean sac diameter [in millimetres]) + 33.117 calculated the gestational age for various mean we 70sac diameters (Table 2). The 95% confidence interval r 0.77 (CI) for the mean gestational age and the 95% prediction interval (PI) for the observed gestational C0 50 age were also calculated (Table 2, Fig. 3). The reliability of the data in Table 2 was determined in 10 women with a singleton intrauter30-...ine pregnancy resulting from in-vitro fertilization. The mean gestational sac diameter was determined - 20 through transvaginal ultrasonography, and the gestaU) 0 10 tional age expected from Table 2 was compared with the actual age. The variability observed between and actual gestational age was 1 day (mean expected 35 20 25 30 15 5 10 0 0.2 0.6] days) (Table 3). [SD Mean sac diameter, mm -
Discussion
Fig. 2: Relation between mean diameter of gestati and menstrual gestational age. Equation for regres: (solid line) was as follows: gestational age (in 0.821(mean sac diameter [in millimetresi) + 35.081 lines represent upper and lower limits of 95% PI. Mear diamete-
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Estimation of gestational age on the basis of knowledge of the last menstrual period is well known to be error-prone. Our results further support this --)rresporlding estimate,s
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observation, since a greater degree of variability was observed when menstrual data were used than when hCG measurements were used. Also, the use of the normal hCG curve to correct for uncertainties in the date of ovulation has been shown to predict gestational age accurately."'12 Our findings confirm that an intrauterine gestational sac can be detected very early with the use of transvaginal ultrasonography and that the hCG threshold level is much lower than previously reported with the use of transabdominal ultrasonography. Because of the small number of cases in which no gestational sac was visualized a practical hCG threshold level could not be determined reliably from our data. Nevertheless, a gestational sac was not observed when the hCG level was less than 1 100
IU/L. This was further reinforced in our subsequent study in which we correlated hCG levels with gestational sac diameter.'3 By extrapolating the curve and calculating the upper 95% PI at a mean sac diameter of 0 mm we found that the hCG level obtained was 1079 IU/L. Similar hCG levels have been reported by others.'4"'5 One group of investigators identified a gestational sac at much lower hCG levels;'6 however, since they did not state which hCG standard was used it is tempting to speculate that it was the 2nd IS.
The fact that during early pregnancy the rate of growth of the gestational sac is linear enables the true gestational age to be calculated. Using the regression equation we tabulated the expected gestational ages so that the mean sac diameter could be converted to the corresponding gestational age. This table was shown to be very reliable and precise in the 70 assessment of pregnancies conceived through invitro fertilization. Therefore, it can be used clinically 60 r 0.95 to determine gestational age. One gold standard used for ultrasonic assessment of gestational age requires la precise measurement of the crown-rump length. This measurement is usually not possible until later cn ~40in the first trimester, when the fetus can be clearly visualized. However, with the use of transvaginal *_ 30 ultrasonography the gestational age can now be accurately determined through the measurement of 0)20the sac diameter. Furthermore, one can monitor a 10 pregnancy by comparing the size of the gestational sac in two ultrasound examinations performed several days apart. This method may provide useful 0 5 10 15 20 25 30 35 prognostic information to women with a history of Mean sac diameter, mm recurrent abortion who are carefully monitored by Fig. 3: Mean diameter of gestational sac and corresponding ultrasonography in early pregnancy and to women estimates of gestational age. Solid line represents mean who are spotting in early pregnancy, especially if gestational age, dotted lines represent upper and lower appropriate growth in sac size is observed. Early identification of intrauterine pregnancy limits of 95% confidence interval, and broken lines represent upper and lower limits of 95% PI. could also improve clinical management of ectopic -
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Table 3: Actual and expected gestational ages in 10 pregnancies conceived through in-vitro fertilization Patient no. 1 2 3 4 5 6 7 8 9 10
Gestational
Mean sac diameter, mm 7 8 17 7 15 9 25 16
Actual 39 40 47 39 46 40 55 46
11 12
44
44
age,* d
Expected 39 40 48 39 46 41 55 47 43 44
Difference 0 0 +1 0 0 +1
0 +1 -1
0
*Actual gestational age was calculated by retrospectively assigning the day of oocyte retrieval as day 14 of gestation. Expected gestational age was obtained from Table 2 after the sac diameter was determined through transvaginal ultrasound scanning. CAN MED ASSOC J 1991; 144 (4)
445
pregnancy. With the use of radioimmunoassay for hCG, pregnancy can be confirmed within 2 weeks after ovulation."7,8 Several investigators have described discriminatory zones using hCG levels to aid in the ultrasonic diagnosis of ectopic pregnancy.2'3 The presence of a yolk sac within the gestational sac in the uterus provides reassuring evidence that there is no ectopic pregnancy. Such a structure could not be visualized with transvaginal ultrasonography when the hCG level was less than 1900 IU/L. In addition, direct visualization of the ectopic pregnancy in the adnexa is now sometimes possible with the use of transvaginal ultrasonography.19,20 Thus, our diagnostic ability has been markedly improved with the advent of transvaginal ultrasound scanning.
References 1. Nyberg DA, Mack LA, Jeffrey RB et al: Endovaginal sonographic evaluation of ectopic pregnancy: a prospective study. AJR 1987; 149: 1181-1186 2. Kadar N, DeVore G, Romero R: Discriminatory hCG zone: its use in the sonographic evaluation for ectopic pregnancy. Obstet Gynecol 1981; 58: 156-161 3. Batzer FR, Weiner S, Corson SL et al: Landmarks during the first forty-two days of gestation demonstrated by the betasubunit of human chorionic gonadotropin and ultrasound. Am J Obstet Gynecol 1983; 146: 973-979 4. Canfield RE, Ross GT: A new reference preparation of human chorionic gonadotropin and its subunits. Bull WHO 1976; 54: 463-470 5. Bangham DR, Grab B: The Second International Standard for chorionic gonadotrophin. Bull WHO 1964; 31: 111-125 6. Bernaschek G, Rudelstorfer R, Csaicsich P: Vaginal sonography versus serum human chorionic gonadotropin in early detection of pregnancy. Am J Obstet Gynecol 1988; 158: 608612 7. Rasor JL, Farber S, Braunstein GD: An evaluation of 10 kits for determination of human choriogonadotropin in serum.
Clin Chem 1983; 29: 1828-1831 8. Kadar N, Romero R: Discriminatory human chorionic gonadotropin zone "demilitarized." Am J Obstet Gynecol 1989; 160: 1255-1256 9. Nyberg DA, Mack LA, Laing FC et al: Early pregnancy complications: endovaginal sonographic findings correlated with human chorionic gonadotropin levels. Radiology 1988; 167: 619-622 10. Daya S: Human chorionic gonadotropin increase in normal early pregnancy. Am J Obstet Gynecol 1987; 156: 286-290 11. Daya S, Woods S, Ward S et al: Early pregnancy assessment with transvaginal ultrasound scanning [abstr 58]. Presented at the 45th Annual Meeting of the Society of Obstetricians and Gynaecologists of Canada, Quebec, June 20-24, 1989 12. Daya S: Determination of gestational age by measuring human chorionic gonadotropin levels. In Boutaleb Y, Gzouli A (eds): The Study of Ovulation and Early Pregnancy (Recent Developments in Fertility and Sterility ser, vol 2), Parthenon, Carnforth, England, 1990: 159-162 13. Daya S, Woods S, Ward S et al: Transvaginal ultrasound scanning and correlation with human chorionic gonadotropin levels. J Clin Ultrasound (in press) 14. Goldstein SR, Snyder JR, Watson C et al: Very early pregnancy detection with endovaginal ultrasound. Obstet Gynecol 1988; 72: 200-204 15. Fossum GT, Davajan V, Kletzky OA: Early detection of pregnancy with transvaginal ultrasound. Fertil Steril 1988; 49: 788-791 16. Timor-Tritsch IE, Farine D, Rosen MG: A close look at early embryonic development with the high-frequency transvaginal transducer. Am J Obstet Gynecol 1988; 159: 676-681 17. Kosasa T, Levesque L, Taymor M et al: Measurement of early chorionic activity with a radioimmune assay specific for human chorionic gonadotropin following spontaneous and induced ovulation. Fertil Steril 1974; 25: 211-216 18. Braunstein GD, Grodin JM, Vaitukaitis J et al: Secretory rates of human chorionic gonadotropin by normal trophoblast. Am J Obstet Gynecol 1973; 115: 447-450 19. Shapiro BS, Cullen M, Taylor KJW et al: Transvaginal ultrasonography for the diagnosis of ectopic pregnancy. Fertil Steril 1988; 50: 425-429 20. Rempen A: Vaginal sonography in ectopic pregnancy. A prospective evaluation. J Ultrasound Med 1988; 7: 381-387
Keeping in fashion I haven't the slightest idea where fashions in pathology are born.... Possibly some of my older readers dimly recollect the days when modish scientists declared that the only dependable method of relieving a toothache was a clean, conclusive appendectomy. Then the whistle blew for the quarter, the two teams changed goals, and it developed that ifyou had a pain in your side it was high time your teeth came out. Late in the second half, tonsils got into the game and broke awayfor a long run; two plays later, colitis recovered a fumble and kicked out of danger, and the issue has been terribly confused ever since. Only one point is clear: it's better to be dead, or even perfectly well, than to suffer from the wrong affliction. The man who owns up to arthritis in a beriberi year is as lonely as a woman in a last month's dress.
Ogden Nash (1902-1971)
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CAN MED ASSOC J 1991; 144 (4)
