Significance of biparietal diameter differences between twins.

Twins of dissimilar size have increased risks both at delivery and in the neonatal period. When dissimilar growth is severe, it should be reflected in birthweight and biparietal diameter (BPD) growth. Sonograms were done on 74 of 106 sets of twins delivered from July 1, 1973, to June 30, 1977. Excluding prematurity and birth trauma, the difference in BPD between twins with normal outcomes in the last scan prior to delivery was 1.32 +/- 0.20 mm (mean +/- SE) and the difference in birthweight was 250.3 +/- 31.4 g (mean +/- SE). In 7 sets of twins with poor outcomes (intrauterine fetal demise or severe intrauterine growth retardation) without central nervous system anomalies the difference in BPD between twins in the last scan, while both fetuses were alive, was 6.86 +/- 1.06 mm (mean +/- SE) and the difference in birthweight was 604.3 +/- 217.2 g (mean +/- SE). Both the BPDs and birthweights are significantly different at the P less than 0.01 level. Large differences in BPD between twins may possibly indicate compromise of one twin.