In 80 consecutive twin pregnancies, prenatal measurements of fetal biparietal diameter (BPD) and abdominal diameter were made and growth curves were calculated using routine ultrasound examinations. Nineteen percent of the infants were growth retarded. Growth retardation was found in both fetuses in four pregnancies and in one fetus in 22 other pregnancies. Linear regression analysis between birth weight and gestational age showed the standard deviation of birth weight to be proportional to gestational age. A more linear growth curve also was found when the mean fetal weight was calculated by use of the BPD and abdominal diameter measurements in the formula developed for singletons. The estimated weight compared with birth weight in 62 twins who had ultrasound examinations less than seven days before delivery showed a significant correlation (r = 0.89, P less than .001) with a coefficient of variation of 12.4%. The identification of intrauterine growth retardation (IUGR) in twin pregnancies by ultrasound had a sensitivity of 62%, a specificity of 98%, and a predictive value of positive and negative test of 93% and 83%, respectively.