For couples having difficulties getting pregnant, the delay in conception may affect their baby's sex

In the study, published in the BMJ, researchers compared information on more than 5,000 Dutch women who gave birth between July 2001 and July 2003.
Among the 498 women who took longer than one year to get pregnant, the percentage of male babies was over 57 percent, compared with 51 percent among the women who took less time to get pregnant.

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.

Because for couples using fertility treatments in this study there was no link between time to pregnancy and the babyÂ’s sex, the authors arehypothesizingg that it is the fact that y bearing sperms may be better swimmers through thick cervical mucus. I would say interesting hypothesis but still untested.