Mothers and Children Share a Birthday Month More Often Than You’d Think

Mother’s pass on all sorts of traits to their children, but a new study finds an unexpected inheritance — birth month.

Mothers are more likely to give birth during their own birthday month, especially when they have multiple children, than at any other time, according to a new study based on birth records from Spain and France. It’s the first time researchers have noticed such a strong correlation, the study authors say, and they hope to replicate the study in other countries. 

The study was published on Thursday in the journal Population Studies.

Since at least the 1990s, researchers had noticed that a pregnant person’s own birthday can somehow influence that of their children, but it wasn’t clear why.

“The decision to conceive is tied to social and demographic factors,” said Luisa Borrell, a social epidemiologist at City University of New York. In the U.S., births spike in August. In Spain, January sees the most new births. In these cases, births might spike in late summer because of social activities around the holidays, or in winter because people tend to get married in early summer. Biological factors, like fertility, can vary throughout the year too.

To find out if there was anything more to the trend, Borrell and her colleagues investigated a dataset of over 10 million births in Spain and France from 1980 to 2013, looking to see if a parent’s birth month influenced when they had kids. Specifically, they analyzed the time distance between both parents’ birth month and their children’s.

“If both mother and child were born in January, the distance is zero,” explained Borrell. “If the mother was born in January and her child was born in February, the distance is one, and so on.”

Curiously, the researchers noticed about 4.6% more instances of the mother and child sharing a birthday month than chance alone would dictate. What’s more, they found siblings shared a birth month about 12% more often than should be expected by chance. 

“We didn’t expect the percentages to be so high,” said Borrell. 

The researchers don’t know why this seems to be the case, but they think families’ shared social and demographic characteristics likely plays a role. In Spain, for instance, highly-educated women are more likely to give birth in the spring. Since the children of highly-educated parents are more likely to become educated themselves, they could exhibit similar patterns, the researchers say.

Other factors, like seasonal temperature, could influence when people decide to conceive, Borrell said, and somehow compound across generations. She emphasizes that more research is needed to fully understand these links.

“It would be great to try to replicate these findings in other countries, like the U.S.”