Both earlier growth charts of twins composed of birth wt at different gestational ages and recent reports on twin biparietal diameter (BPD) growth indicate that twins'' growth is less than that of singletons during the later period of gestation. Growth curves were constructed separately for the 1st and the 2nd twin from 489 paired ultrasonic BPD measurements of 119 twin pregnancies. The distribution of BPD values in twins was slightly wider than the normal range curve for singletons. The mean BPD values of twins were close to those of singletons in the 2nd trimester. From the 32nd wk on the twins'' mean weekly BPD increment decreased, this lesser growth rate being more marked than that of singletons. The 2nd twin''s BPD growth curve ran consistently below that of the 1st. The growth of twins apparently is retarded during the later part of pregnancy. Differences in BPD growth patterns between the 1st and the 2nd twin are indicated.