Does the time of the month that a baby is conceived affect if it’s a boy or a girl?
It’s possible, says British mathematician Hannah Fry.
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“The chances of a woman conceiving a male or female child actually very subtly changes depending on when in her cycle she conceives,” Fry explained in a video on her Instagram page. “So slightly earlier, ever so slightly more likely that she’ll have a baby boy.”
However, Fry said the odds are “way too small” for couples to use that as a predictor.
The larger point of her investigation was analyzing gender trends over time. With data focusing on England and Wales, she explored something called the “returning soldier effect,” which shows spikes in male birth rates following World War I and II.
The trend, which has also been observed in other countries, has several possible explanations.
One controversial theory is that healthier soldiers are more likely to survive war and come home. That, combined with the Trivers-Willard hypothesis —which posits that healthier people are more likely to conceive boys—could explain the spike.
Another hypothesis is that the increase in male births is nature’s way of compensating for the loss of life during the war.
The ‘returning soldier effect’ is just one possible explanation for birth rate trends
Fry admits that there’s no way to tell for certain what causes more male births than female, or vice versa. There are also dozens, if not hundreds, of other factors that come into play, including the health of the mothers and environmental conditions.
For example, writing in 2008, William H. James observed a decline in male birth rates in Iran during and following the Iran-Iraq war. He attributed the fall to “psychological stress causing pregnant women disproportionately to abort male fetuses.”
However, the “returning soldier effect” definitely has a basis in fact.
“At the end of a war, everyone has much more sex than normal,” Fry noted. “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.”