Sonar cephalometry in twins: a table of biparietal diameters for normal twin fetuses and a comparison with singletons.

In 123 normal twin pregnancies, 589 biparietal diameter (BPD) measurements were obtained between 16 and 40 weeks' gestation and mean values for each week were computed. A table of BPDs for normal twin pregnancies based on these data is proposed. Mean twin BPDs were consistently smaller than those of singletons, the difference averaging 3.5 mm between 16 and 40 weeks' gestation. The development of a twin BPD table now permits a more accurate assessment of twin gestational age and fetal growth.