A study about the sex prevalence performed in The Netherlands found that women who took longer to get pregnant were more likely to give birth to a boy.
Researchers at Maastricht University looked at 5,283 women and results showed that around 58 % of women who had tried to conceive naturally for over a year had a boy, compared to 51% who became pregnant in a shorter time. Each year of trying gave couples a 4% higher chance of having a boy. The same results were given when age, smoking and alcohol use were taken into account.
The theory behind this finding is that women whose cervical mucus is relatively thick would have more problems conceiving naturally because of the difficulties faced by the sperm. It is thought that male sperms were better able to cope with the thicker mucus because Y chromosome sperms are able to swim faster than X chromosome sperms in thick, viscous fluids.