Background: The existence of a male excess among preterm births is interesting because it could shed light on the aetiology of preterm birth. Possible mechanisms are greater body weight, increased susceptibility to complications of pregnancy, sex-linked biochemical processes and earlier conception in the fertile cycle.
Methods: We measured the association between fetal sex and preterm birth in four original datasets, including a cohort of births after IVF, and 20 populations extracted from published birthweight references. The original samples were also analysed by mode of onset.
Results: There were more males among preterm and early preterm births than among term births in most populations, including IVF births (odds ratio: 1.09-1.24). No male excess was observed for two cohorts of black births, induced preterm births in the general population, and spontaneous onset births after IVF.
Conclusions: The proportion of male births declines with increasing gestation, even when time of conception is known. This male excess appears to be strongest for spontaneous preterm births. Studying the sex ratio of preterm births by medical risk factors may clarify why the male excess is absent in some populations. The possibility that obstetric decision-making affects the sex ratio of indicated births must be considered.