The relationship between gestational age and birth-weight in twin pregnancy.

Analysis is made of fetal growth in 563 twin pregnancies. The birth-weight of a twin is affected by the duration of pregnancy and zygosity. Intrauterine growth and weight is the same as a singleton pregnancy up to a gestational age of 32 weeks. After this stage fetal growth slows markedly. The growth curves of both twins are similar with an increasing tendency towards growth retardation in twin 2 after 39 weeks' gestation. Intrauterine growth retardation, as represented by the 10th percentile line, for twin 1 joins 1,094g at 31 weeks, 1,836g at 36 weeks and 2,428g at 40 weeks' gestation. The corresponding figures for twin 2 are 1,131g, 1,875g and 2,337g respectively.