Couples are more likely to have baby boys than girls when they have sex at this time of the month

Oh, boy!

There’s no way to guarantee you’ll conceive a baby boy — short of implanting a male embryo created through in vitro fertilization — but that hasn’t stopped many couples from trying anything (and everything!) to make that dream come true.

Some have timed intercourse close to ovulation, while others have engaged in deep, penetrative sex. Some women have increased their potassium intake, while others have consumed a lot of calories.

British mathematician Hannah Fry suggests getting out your calendar — and looking to history.

On Instagram this week, she pointed out that historical data show a noticeable spike in the number of boys born in England and Wales following World War I and World War II — a pattern observed in other countries as well.

This phenomenon is known as the “returning soldier effect” — several theories attempt to explain it.

One is that healthier male soldiers are more likely to survive war and come home to their wives — and healthier people are more likely to produce male offspring. That’s the basis of the controversial Trivers-Willard hypothesis, which has shown mixed results across different species and studies.

Another rationale for the increase in baby boys is that nature is compensating for the loss of men in combat, ensuring the population has enough males to reproduce.

And finally, there’s the idea that soldiers immediately engage in sexual activity upon returning home, causing women to get pregnant earlier in their cycles.

“The chances of a woman conceiving a male or female child actually very subtly changes depending on when in her cycle she conceives,” Fry said. “So slightly earlier, ever so slightly more likely that she’ll have a baby boy.”

Unfortunately for couples hoping to conceive a boy, Fry lamented that these odds are “way too small” for an individual couple to “game” them.

“But when you scale up to the size of a population, this is a pattern that becomes really clear. At the end of a war, everyone has much more sex than normal,” Fry said. “Women get pregnant slightly earlier in their cycles, and as a result, there is a spike in the number of baby boys that are born.”

Studying the “returning soldier effect” has proven difficult because it’s tough to isolate the soldiers’ biology from other factors, such as the health of the mother and environmental conditions. 

Meanwhile, in the US, the birth rate for boys has been consistently slightly higher than the birth rate for girls.

In 2022, for example, there were 1.048 male births for every female birth. No intervention was necessary.