We studied 153 consecutive twin pairs of unlike sex. Comparison of 71 male-female pairs with 82 female-male pairs showed similar mean twin birth weight and rate of discordance. However, males were significantly heavier (p < 0.04) compared with females in the male-first but not in the female-first combination. The rate of female discordance was significantly higher compared with male discordance in both fetal sex combinations. It is concluded that females of unlike-sexed pairs are at greater risk to be growth discordant. Fetal sex, therefore, should be included as one of several factors that produce divergent twin growth.
- birth weight