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ORIGINAL RESEARCH |
From the Division of Maternal Fetal Medicine, Department of Obstetrics and Gynecology, and the Department of Biometry and Epidemiology, Medical University of South Carolina, Charleston, South Carolina.
Address reprint requests to: Jill G. Mauldin, MD Department of Obstetrics and Gynecology Medical University of South Carolina 96 Jonathan Lucas Street, Suite 634 P.O. Box 250619 Charleston, South Carolina 29425 E-mail: mauldinj{at}musc.edu
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Abstract |
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Methods: Using an established database, we reviewed 338 twin gestations delivered over 10 years as a retrospective cohort. Estimation of fetal weight was calculated by applying the Hadlock formula using composite fetal biometry. Intertwin weight discordance was calculated as the difference in the estimated or actual twin weights (A–B) divided by the weight of the larger twin and was expressed as a percentage. Statistical evaluation included validity (sensitivity, specificity, and predictive values) and reliability assessment of ultrasonographic measurements (intraclass correlation coefficients). Multivariable analysis was performed.
Results: Of 338 twin gestations, 192 (57%) twin pairs met inclusion criteria. Sonographic prediction of actual intertwin birth weight discordance of 25% or greater had a sensitivity of 55%, specificity of 97%, positive predictive value of 82%, and negative predictive value of 91%. The reliability of estimating intertwin birth weight discordance by ultrasonography was moderately high (intraclass correlation coefficient = .700; 95% confidence interval [CI] .620, .765). Multivariable analysis revealed no significant effects of individual maternal or fetal factors on the accuracy of ultrasonographic prediction of intertwin birth weight discordance.
Conclusion: Sonographic prediction of actual intertwin birth weight discordance of 25% or greater within 16 days of delivery appears to be a valid and reliable method for clinical use. Predictive accuracy is independent of other identifiable maternal or fetal variables.
The incidence of twin birth has increased approximately 33% over the past 20 years, and twin gestations now account for one of every 45 live births.1 This increased frequency of twin birth is attributable in part to use of ovulation-induction agents and assisted reproductive technologies and to a trend toward delayed childbirth. Multiple gestations, however, are not without risks. Fetal growth abnormalities, along with preterm delivery, are the major contributors to increased rates of perinatal morbidity and mortality associated with multiple gestations.2
Significantly discordant fetal growth has been reported to occur in 15–30% of twin gestations. The incidence of discordance is directly related to the definition used.3 When birth weight discordance exceeds 25%, perinatal mortality increases twofold and measures of perinatal morbidity increase sixfold.4 In addition, these infants often have continued growth disparity throughout childhood and the smaller twin is at increased risk for long-term neurodevelopmental delays.5–7
Antepartum and intrapartum management plans are significantly influenced by sonographic estimation of fetal weight and intertwin growth discordance. For instance, antenatal detection of severe growth discordance (25% or greater) typically results in initiation of fetal surveillance protocols or decision to deliver. At the time of labor, estimations of fetal growth and percent discordance direct patient counseling and discussions about possible interventions and delivery routes.8
We sought to assess the accuracy of sonographic prediction of intertwin birth weight discordance and to determine whether this accuracy was affected by maternal and fetal variables.
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Materials and Methods |
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Inclusion criteria were live birth of both twins at or beyond 24 weeks’ gestation, birth weight of 500 g or greater, and sonographic prediction of estimated fetal weight and percent discordance performed within 16 days of delivery. Ultrasonography was performed by one of seven certified registered diagnostic medical sonographers using Acuson-128 XP and Sequoia Imaging Systems (Acuson, Mountain View, CA) or the GE 3200 (General Electric, Norcross, GA) ultrasonograph. Each scan was reviewed by a Maternal Fetal Medicine faculty member.
Estimated fetal weight was calculated by applying the Hadlock formula using composite fetal biometry, including head and abdominal circumferences, biparietal diameter, and femur length (FL).9 Fetal discordance was calculated by determining the difference between the estimated fetal weights of twin A and B and dividing that value by the estimated fetal weight of the larger twin; this result was expressed as a percentage. Actual intertwin discordance was calculated by using the same method but with the true birth weights of each twin. Discordance was considered present if the calculated percentage was 25% or greater and absent if the percentage was less than 25%.
Statistical evaluation included determination of sensitivity, specificity, and positive and negative predictive values of sonographically determined percent discordance. The relation between the estimated and actual twin birth weights and intertwin discordance were assessed by using the intraclass correlation coefficient.
Multiple linear regression analysis was used to assess individual effects of multiple maternal and fetal variables on the accuracy of sonographic prediction of intertwin discordance. Maternal variables were height, weight, body mass index (BMI), gravidity, parity, mode of conception, chronic hypertension, diabetes, anemia, tobacco use, alcohol use, and presence of uterine tumors (leiomyomas). Fetal variables were gestational age, fetal sex, the twin–twin transfusion syndrome, fetal presentation, fetal growth restriction (FGR), and oligohydramnios. Statistical significance was set at P < .05.
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Results |
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). Neither reducing the number of days between ultrasonography and delivery nor reducing the criteria for significant discordance to 20% or greater affected the sensitivity or specificity of ultrasonographic diagnosis (Table 3).
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Multivariable analysis was performed on 149 pregnancies for which data on each independent variable were complete. We determined the individual effects of these maternal and fetal variables on the accuracy of sonographic prediction of intertwin birth weight discordance. Because of the number of independent variables, we performed forward stepwise regression analysis. Neither maternal nor fetal characteristics significantly affected (P > .05) the difference between predicted and actual intertwin discordance measurements.
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Discussion |
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).12,14,15
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We reanalyzed the data using a shorter interval between ultrasonography and delivery and redefined intertwin discordance as a growth disparity of 20% or greater instead of 25% or greater. The samples for secondary analyses were smaller and therefore more difficult to interpret. Changing the inclusion criteria from the original analysis did not significantly affect sensitivity or specificity.
The positive predictive value that we found is high compared to previous reports, and the wide CI is a concern. Because suspected significant intertwin growth discordance carries potentially important management implications, the diagnosis must be reliable. In more than four of five cases, the antepartum diagnosis of intertwin discordance of 25% or greater was correct. However, it is important to note that significant discordance may be overdiagnosed in up to 20% of cases. Decisions about elective preterm delivery because of intertwin growth discordance should be made carefully and the entire clinical picture should be taken into account. Similarly, the fact that the sensitivity of ultrasonography for significant intertwin discordance is only 60% at best indicates that routine antepartum fetal surveillance should be considered in all multifetal gestations near term.
An inherent limitation of this investigation is its retrospective nature. Potential confounding variables may not have been controlled for adequately. Some investigators may view the relatively large number of sonographers as a limitation against achieving higher sensitivity. Others would believe that this is consistent with the structure of current physician practices.
Another possible confounding variable is the 16-day interval between ultrasonography to estimate discordance and delivery. This interval may have allowed for significant fetal growth, thereby altering the accuracy of ultrasonographic prediction. Although prediction estimates of greater than 16 days were not explored, the results did remain consistent when the interval was shortened.
The number of twin gestations in our study is twice as large as the number studied by Blickstein et al15 and Chamberlain et al11; however, the relatively small sample associated with calculations of validity and reliability may be a limitation. In particular, the samples used to estimate sensitivity and positive predictive value (33 and 22, respectively) may have contributed to the wide CIs. Although more twin data must be collected to confirm our conclusions, our findings show that ultrasonography is a promising tool for estimating intertwin birth weight discordance.
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Footnotes |
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Received March 13, 2000. Received in revised form July 18, 2000. Accepted August 10, 2000.
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References |
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2. Bronsteen R, Goyert G, Bottoms S. Classification of twins and neonatal morbidity. Obstet Gynecol 1989;74:98–101.
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10. Crane J, Tomisch P, Kopta M. Ultrasonic growth patterns in normal and discordant twins. Obstet Gynecol 1980;55:678–83.
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  D. Danon, N. Melamed, R. Bardin, and I. Meizner Accuracy of Ultrasonographic Fetal Weight Estimation in Twin Pregnancies Obstet. Gynecol., October 1, 2008; 112(4): 759 - 764. [Abstract] [Full Text] [PDF]
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