Ultrasonic growth patterns in normal and discordant twins.

Prior studies on twin biparietal growth have resulted in conflicting conclusions. This study was designed to establish definitive criteria based on both ultrasonic biparietal diameter (BPD) and newborn anthropometric data for normal versus abnormal growth in multiple pregnancies. Fetal BPD and newborn head circumference values among concordant twins were similar to those of appropriate for gestational age (AGA) singletons at all gestational ages. Singleton standards can therefore be applied to evaluate the adequacy of twin BPD growth even in the third trimester. Ultrasonic criteria for the diagnosis of discordancy include an intrapair difference in BPD of 5 mm or more and an intrapair difference greater than 5% in ultrasonically measured fetal head circumferences. Discordant BPD growth noted prior to the third trimester suggests twin-transfusion syndrome and is associated with a high perinatal mortality. In contrast, divergent BPD growth beginning after 30 weeks' gestation appears to have a better prognosis.