If you have a brood of girls and you’re trying one last time to have a boy, you’ve probably heard a lot of urban myths about how to increase your chances of having a boy. Before we even look at the studies, just remember that the sex of your baby is determined solely by the sperm. If the sperm has a Y chromosome, then the baby will be a boy. If the sperm has an X chromosome, then the baby will be a girl. What it comes down to is how (and if) you can help the sperm with the Y chromosome win the race to fertilize the egg.
While your grandmother or that nice lady down the street might have some ideas about sure-fire ways to have a boy, scientists have really only debated about one method of sex selection in particular. Several studies have looked at whether there is a connection between the sex of the baby and the time of the month when the egg is fertilized. In particular, scientists have investigated whether the sex of the baby can be determined based on the time between when you have sex and when you ovulate. Researchers suggested that you could increase your chance of having a boy if you had sex very close to the time when the woman ovulated. The mucus from a woman’s cervix changes during a woman’s cycle, with the mucus getting thicker during the middle of the cycle, around the time of ovulation. They theorized that you might be more likely to get boys when the mucus is thicker (close to the time of ovulation) because sperm with a Y chromosome are more mobile and thus more likely to penetrate that mucus than the sperm with an X chromosome. And some of the studies testing out this theory reported slightly more boys conceived during sex close to ovulation, enough to show a small but statistically significant difference in whether you get girls or boys from sex at different times of the month.
Unfortunately, the more scientists study this, the less consistent the results. In a meta-analysis that combined all of the data from six studies that looked at the timing of intercourse and whether you ended up with a boy or a girl, researchers actually found fewer boys among those conceived during the most fertile time of the cycle (right around ovulation). The differences were still small, but completely the opposite of the aforementioned findings. In another study of the sex-selection technique involving seventy-three women over four years, there was no difference in the number of boys or girls among those who tried to select the sex of the baby by timing their intercourse versus those who did not.