Sonographic evaluation of growth discordance and chorionicity in twin gestation

Ninety-four twin gestations had serial ultrasound examinations for evaluation of fetal growth, with special attention to prediction of weight discordance and chorionicity. Fetal weight estimations were made a mean of 3.2 days before birth and then compared to actual birthweights. The average error in intertwin discordance prediction was 1 +/- 9%. In 56 patients ultrasonic detection of membrane thickness was compared with placental pathologic conditions. If a membrane was seen between twins, it was confirmed by placental pathologic study in all cases. No membrane was seen in four twin pairs, but only one of these actually had monochorionic monoamniotic membranes. The three cases with false-positive diagnoses of monoamniotic twin gestation were initially seen late in pregnancy. If clinical decisions in twin gestations are based on ultrasound determination of discordance and chorionicity, the level of accuracy demonstrated by these data must be considered.