Birthweight of full-term twin infants in relation to sex-pair

The present study examined the effect of sex-pair on birthweight of twins. The birthweight and sex-pairs of 104 live-born full-term twin pairs were recorded and analyzed. The mean birthweight and the mean birthweight sum of a different-sex twin pair were compared with that of a same-sex twin pair. The relative incidence of low birthweight (LBW) and birthweight discordance in the different-sex twin pair and same-sex twin pair were examined. Comparing the mean birthweight of the different-sex twin pair with that of same-sex twin pair, it was 2503 ± 428 g [95% confidence interval (CI) = 2369–2637] versus 2398 ± 442 g (95% CI = 2291–2505), p > 0.05. The overall mean birthweight sum of the different-sex twin pair was 3889 ± 492 g (95% CI = 3679–4099) while, for the same-sex twin pair, it was 3665 ± 512 g (CI = 3493–3837), p > 0.05. The mean birthweight sum for the male-male twin pair was 3903 ± 536 g (95% CI = 3648–4158), while for the female-female twin pair it was 3426 ± 560 g (95% CI = 3160–3192); p < 0.05. The mean birthweight of female infants in the male-female twin pair was 2081 ± 492 g (95% CI = 1927–2235) compared with 1795 ± 502 g (95% CI = 1635–1955) for female infants in the female-female twin pair; p > 0.05. The figures for the male infants in the male-female pair and the male-male twin pair were 2136 ± 468 g (95% CI = 1989–2283) and 1976 ± 519 g (95% CI = 1804–2148), respectively; p > 0.05. Rate of delivery of LBW twin pairs in all live-born twins were: different-sex, 51.3%; male same-sex, 43.8%; and female same-sex, 75.8%. Of the 59 LBW twin pairs, 20 (33.9%) were different-sex and 39 (66.1%) were same-sex twin pairs. In conclusion, cohabitation in the uterus of twin fetuses of different sexes influenced their intrauterine growth.