Sonar cephalometry in twin pregnancy: discordancy of the biparietal diameter after 28 weeks' gestation.

Sonar measured biparietal diameter (BPD) differences of twin paires were examined in 123 twin pregnancies at or beyond 28 weeks' gestation. Among 117 liveborn sets, the risk of a twin infant being small for gestational age was threefold greater when paired BPD differences were 5 mm or more compared to 4 mm or less. The incidence of fetal death increased from 2.7% for twin pairs with 0 to 6 mm BPD differences to 20% when the difference was 7 mm or more. Sonar cephalometry may be helpful in the antepartum evaluation of twin pregnancies, although detection of BPD discordancy does not preclude normal twin outcome.