Do you share your birthday month with your mom or child? It's a surprisingly common phenomenon, according to a new recent study that uncovered patterns in human birth seasonality. It turns out that women are more likely to have children in the same month as their birth, and births within immediate families tend to cluster within a given month.

Researchers made the findings after evaluating data from more than 10 million deliveries in France and Spain – all births from 1980 to 1983 and from 2016 to 2019 in Spain, and all births from 2000 to 2003 and from 2010 to 2013 in France. The results were published in the journal Population Studies.

"Our results showed the existence of a similarity not only between parents' and children's birth seasons but also between the two parents' birth seasons, and between adjacent siblings' birth seasons; this partially explains the stability of seasonal patterns over time," the researchers wrote.

They observed that births within a particular country tend to follow a pattern, where certain times of the year have more births than others. This seasonal variation in human birth rate is known as birth seasonality.

However, when the birth data was grouped based on the month of birth of the mothers, the researchers did not see the expected pattern. Instead, there was a spike in births in January among the group of mothers born in January and so on. There were 4.6% more births than expected in which the mother and child shared the same birth month. The result was consistent in both countries and all four time periods examined for the study.

"It was also the case for siblings (there were 12.1% more births than expected in which adjacent siblings had the same month of birth), parents with the same month of birth (4.4% more births) and when a child had the same birth month as their father (2% more births)," the researchers said in a news release.

The team attributes the phenomenon to shared socio-demographic features of family members. Factors such as the availability of food and exposure to sunlight affect a person's fertility. People with similar backgrounds are known to pair up and are more likely to give birth at certain times of the year, researchers explained.

"What could cause the higher probabilities of family members being born in the same season? The potential explanations seem to be both social and biological," said study author Adela Recio Alcaide, an epidemiologist at the University of Alcala.

"We discuss the tendency of family members to be born in the same season as the result of two facts: first, that different socio-demographic groups show differentiated birth patterns and, second, that relatives share socio-demographic features. Finally, we conclude that birth season is related to family characteristics and that these should be controlled for when assessing birth-month effects on subsequent social/health outcomes," the researchers noted.

A limitation of the study is that the analysis assumes an "independence of outcomes," but researchers caution that there is a "dependence of the outcomes within families," which could have affected the findings. However, when the team repeated the analysis after adjusting for this factor, the results were very similar.

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