Do you and your kids share a birthday month?
The idea is not that strange, according to a new study published in the journal Population Studies. In fact, there's a scientific reason for it!
Children and their mothers celebrate their birthdays in the same month more often than could be explained by chance, found the study; so do siblings. Fathers experience the same effect with their children's birthdays, to a slightly lesser degree than mothers. Research also found that sets of parents are often born during the same month.
Seasonal patterns aside, researchers found that mothers born in May delivered more babies in May; mothers born in June delivered more babies in June, with the pattern reflective month over month. They looked at 10 million births in Spain and France during time periods spanning from 1980 to 2019.
"Although research on birth seasonality has shown that women’s season of birth somehow influences that of their children, the mechanisms of these relationships have not been identified," wrote the study authors. "Not only did we find that women’s own season of birth influenced the seasonal patterns in the births of their children, but we have clarified how this influence works: women are more likely to have children in the same month as their own birth (with a 4.6 percent excess of births where birth months coincided, on average across periods and countries studied)."
Other similarities popped between other family members.
Parents shared a birthday month more often than would be expected just by chance. Researchers explained there are a lot of reasons why this would be: Birth patterns tend to be effected by things like seasons, economics and holidays.
"Why are individuals more likely to choose a partner born in the same month as themselves?" wrote the researchers. "This may not be surprising considering two facts: first, partnerships tend to be formed by people with similar socio-demographic characteristics and, second, evidence has shown on numerous occasions that different socio-demographic groups display different seasonal birth patterns."
Families who share birthdays, even down to the date, have drawn public interest.
One Florida mom gave birth to her three children on Aug. 25, three years apart, an "extremely rare event," Dr. Christine Greves, an OB-GYN at Orlando Health Winnie Palmer Hospital for Women & Babies, told TODAY.com.
Another family in Alabama (mom, dad and baby) share the same Dec. 18th birthday.
Huntsville Hospital for Women & Children in Huntsville, where the baby was born, called the occurrence "a chance that’s one in 133,000."