This study aims to determine whether fetal growth is related to insulin-like growth factor-1 in dichorionic and monochorionic twins and also aims to investigate the correlation of insulin-like growth factor-1 to birthweight discordance in twins. We studied 100 women with twin pregnancies. The correlation tests of 36 discordant twins (15 monochorionic, 21 dichorionic) showed correlation between insulin-like growth factor-1 difference and birthweight discordance (insulin-like growth factor-1 vs. birthweight of first twin, r = +0.915, at 0.01 level, IGF-1 vs birthweight of second twin r = +0.790, at 0.01 level). In 49 monochorionic twins, independent of discordance, there was a correlation between birthweight discordance and insulin-like growth factor-1 difference (r = .538, at the 0.01 level). This correlation was not significant in dichorionic twins, r = .144, p = .01. These data suggest that growth discordances of twins exposed to the same maternal environment may be due to variations in IGF-1, depending upon the genetic similarity.