Ninety-six fetuses of uncomplicated concordant twin pregnancies were monitored by ultrasonic cephalometry from 13 to 38 weeks gestation. The biparietal diameter (BPD) growth of 96 twin fetuses was then compared to a sample of 299 uncomplicated, appropriate-for-gestational-age singleton pregnancies. Regression analysis applied to these measurements of BPD on twin and singleton pregnancies showed no significant difference in biparietal diameter between uncomplicated singleton and twin pregnancies. It is concluded that charts derived from singleton pregnancies may be reliably used for estimation of gestational age of twins.