Surprise data shows parents more likely born in same month

Do you celebrate your birthday in the same month as someone else in your immediate family? If so, you’re not alone.

A new paper has found that the phenomenon occurs more often than you would expect.

It is the first to show that women are more likely to give birth to children in the same month as they were born. Siblings also tend to share the same birth month, as well as children and their fathers.

The trend even extended to the parents, with mothers and fathers sharing the same month of birth as each other in 4.4% more births than would be expected.

The team of researchers from Spain and the US analysed data on more than 10 million births in Spain and France. The records provided information on the child’s month of birth, both of their parents, and the sibling that was closest to them in age.

Overall, there were 4.6% more births in which mothers and their children shared a birth month than would be expected. There were also 12.1% more births in which adjacent siblings shared the same month of birth, and 2% more births in which a child had the same birth month as their father.

“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,” says Dr Adela Recio Alcaide, an epidemiologist at the University of Alcalá, Spain, and co-author of the study.

“The excess of children with a father and mother born in the same month seems to be due to social or behavioural causes prior to conception that relate to the choice of a partner born in the same month, as we have observed this excess with marriage statistics, with spouses being more likely to mate with someone from the same month,” says Alcaide.

Co-author Professor Luisa Borrell, from The City University of New York in the US, adds: “This may not be surprising, considering things such as partnerships tend to be formed by people with similar socio-demographic characteristics.”

People with similar backgrounds are known to pair up more often. They are also more likely to give birth at certain times of the year.

For example, a Spanish woman with a higher education is more likely to give birth in the spring, compared to a woman without a higher education. Her daughter may also be more likely to have a higher education, because her mother does, and will therefore also be more likely to have her children in the spring too.

“Moreover, biological factors that are known to affect birth seasonality – such as photoperiod exposure, temperature, humidity, and availability of food – also depend on socio-demographic characteristics, since different social groups are exposed to these biological factors to varying degrees,” adds Borrell.

“This study has uncovered what until now remained unclear in the field of human birth seasonality: the tendency of members of the same family to be born in the same season of the year, which implies the transmission of seasonal birth patterns between generations,” the authors write in the study, which has been published in the journal Population Studies.