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The sonographic assessment of twin growth discordancy
L M Hill1, D Guzick, P Chenevey
1Department of Obstetrics, Gynecology, and Reproductive Sciences, Magee Womens Hospital, University of Pittsburgh School of Medicine, Pennsylvania.
Objective:
To determine: 1) the frequency with which standard fetal biometry (head circumference [HC], abdominal circumference [AC], and femur length [FL]) and the transverse cerebellar diameter can be measured in twin pregnancies; and 2) the efficacy of fetal biometry using these measures in the detection of twin growth discordancy.
Methods:
The study population consisted of 203 twin pregnancies reviewed retrospectively. The frequency with which standard biometry and the transverse cerebellar diameter could be obtained was recorded. Forty-nine twin pairs who were delivered within 3 weeks of their last ultrasound examination were divided into three groups based on birth weight differences: 20% or more, 10-19%, and less than 10%. The sensitivity, specificity, and predictive values of the specific fetal biometric measurements and of the sonographic estimation of fetal weight were assessed for this subgroup for the prediction of twin discordancy.
Results:
The FL could be measured consistently throughout gestation, but the ability to measure the AC decreased after 35 weeks' gestation. The frequency with which HC and transverse cerebellar diameter could be measured decreased with advancing gestation. The transverse cerebellar diameter could be measured only in 91 of 151 and 14 of 49 pregnancies at 31-35 and 36-40 weeks' gestation, respectively. An intra-pair AC difference of 20 mm or more had a sensitivity and a positive predictive value of 83% for the detection of twin discordancy, defined as at least a 20% difference in birth weight. Estimated fetal weight had a sensitivity and positive predictive value of 92.9 and 72%, respectively. In contrast, a difference of 4 mm or more in the intra-pair transverse cerebellar diameter had a sensitivity of 28% and a positive predictive value of 50% in detecting twin weight discordancy.
Conclusions:
Appropriate twin fetal biometry can be measured throughout gestation. Estimated fetal weight has a higher sensitivity but a lower positive predictive value than AC for predicting twin growth discordancy.