Effect of Co-twin Fetal Sex on Fetal Anthropometry and Birth Time in Twin Pregnancies

This study of twin deliveries aimed to examine the effect of fetal sex and fetal sex of the co-twin on fetal anthropometry and length of gestation.Pregnancies were grouped as male/male, male/female, and female/female. Birth weight, head circumference, body length and delivery time of newborns were compared between unlike-sex and like-sex twin pregnancies.A total of 1028 pregnant women who met the inclusion criteria were enrolled in the study. Of these pregnancies, 32.6% (n = 335) were male/male, 33.4% (n = 343) were male/female, and 34.0% (n = 350) were female/female. The discordant (male/female) newborns had a higher total birth weight than concordant twins (P = 0.015). Compared with male newborns from male/female twin pregnancies, male newborns from male/male pregnancies were found to be 129 grams heavier, 0.7 cm longer, and had a 0.4 cm larger head circumference (P0.001, P=0.023, and P = 0.039, respectively). Pregnancies with male/female fetuses had statistically significantly longer gestations than pregnancies with male/male and female/female fetuses (P = 0.003 and P = 0.004, respectively). The shortest mean gestation was observed in the male/male group. Male/male pregnancies had a 1.53 times higher risk of preterm delivery than male/female pregnancies and a 1.51 times higher risk than female/female pregnancies (OR 1.53; 95% CI 1.07-2.19 and OR 1.51; 95% CI 1.06-2.16, respectively).This study suggests that, in twin pregnancies, birth weight, head circumference, and body length are affected by the sex of the co-twin. Male sex is associated with shorter gestation and male/male twin pregnancies are at higher risk for preterm labour.