Time to pregnancy and sex of offspring: cohort study.

The proportions of X and Y chromosome bearing sperms in human semen are equal, but more boys than girls are born.Male embryos and fetuses have a greater risk of attrition in utero than their female counterparts, and therefore male excess is likely to be still larger at the time of conception. It remains unexplained, however, what is responsible, presumably at some point between insemination and conception, for the greater probability of Y bearing sperms fusing with the ovum. One hypothesis relates to experiments showing that Y bearing sperms swim faster than X bearing sperms in viscous fluids. For natural conception, human sperms have to penetrate cervical mucus, the viscosity of which varies among and within women. Since mucal viscosity also influences the probability of conception, we expected that natural conceptions that take longer to achieve are more likely to be male than quick conceptions. We tested our prediction by assessing the relation between time to pregnancy and sex of the offspring.