Twin pregnancies (28) were monitored by serial ultrasonic cephalometry from 30 or 31 wk gestation. The rates of growth of the individual twins as determined by biparietal diameters were similar in 11 cases (39%) and divergent in 17 (61%). When the rates of growth were divergent, the lesser rate was always below the mean for singleton pregnancies and the incidence of small-for-gestational-age babies was 18 of 34 (53%). The greater the difference in biparietal diameters within the 2 wk preceding delivery, the higher the risk of a small-for-gestational-age baby being delivered. No comment was made on the growth rate prior to 28 wk except that at diagnosis there was little or no difference in biparietal diameters.