Objective: To test the hypothesis that the presence of a male affects birth weight of its female co-twin.
Patients and methods: We evaluated a large population dataset of bichorionic twins after exclusion of potential confounders and after controlling for parity and gestational age. We tested the hypotheses that males (M) are heavier than females (F) irrespective of gender mix, parity and gestational age, and the hypothesis that birth weight of twins might be influenced by the gender of the co-twin.
Results: There were 819 MM pairs, 777 FF pairs and 1097 MF sets, for a total of 2735 males and 2651 female twins. Male twins were heavier than female twins, irrespective of parity or gestational age; twins born to multiparas were heavier than twins born to nulliparas, except for very preterm births (≤32 weeks); males from MF pairs were heavier than males from MM pairs, but the mean birth weight of females from MF was not different from that of females from FF sets suggesting no "masculinization" effect of the male on birth weight of its female co-twin, irrespective of parity and gestational age.
Conclusions: After exclusion of potential confounders and controlling for chorionicity, parity, and gestational age, our data do not support the presence of a "masculinization" effect on birth weight.