The intrauterine growth of the biparietal diameter of twins

A series of 978 repeated ultrasonic measurements of the fetal biparietal diameter (BPD) assessed the growth of 119 twin pairs during the second half of pregnancy. The mean BPD values for each gestational week were close to the mean values for singletons. From the 32nd week, the BPD growth curves for twins deviated from that for singletons. The second twins' growth curve ran consistently below that of the first and fell away more from the 32nd week on. In 80% of the measurements, the first twin (defined as the twin in, or close to, the pelvic inlet) was the one with the largest BPD. The intrapair difference in BPD had no correlation to the intrapair difference in body length, head circumference, or birth weight. In 91 of the 119 twin pairs, zygosity was settled by sex, histological examination of placenta, or blood grouping. The longitudinally compiled BPD growth curves (678 BPD determinations) showed the curve for dizygotic twins consistently running above that of the monozygotic twins. The BPD differences between the first and the second twin remained also within mono- and dizygotic groups of twins.