Time to pregnancy and sex of offspring: cohort study — Smits et al. 331 (7530): 1437 — BMJ
We analysed data of 5283 Dutch women who gave birth to singletons between July 2001 and July 2003. All of the women, at about 14 weeks of pregnancy, were recruited by midwives and gynaecologists (response rate 5283/7200, 73.4%). Mean age of the women at the time of conception was 30.5 years; 83% of their pregnancies were planned, and 47% were delivering their first baby.
Among the 498 (9.4%) women with times to pregnancy longer than 12 months, the probability of male offspring was 57.6% (287), whereas the proportion of male births among the 4785 women with shorter times to pregnancy was 51.1% (2445, 2 = 7.81, P = 0.0052). The proportion of male offspring after different times to pregnancy in the 4982 couples with natural conceptions is shown in the figure.
１年以上たって妊娠した場合58％が男の子、１年以内の場合は51％が男の子。