Scientists believe they may have the answer to why so many mothers share the same month of birth with their children.

An analysis of more than 10 million births in Spain and France has shown it is more common than previously thought for family members to have birthdays in the same month or during the same season.

Births in different countries usually follow a pattern, scientists say, with more babies being born at certain times of the year than at others.

But when the team analysed the data based on the birth month of the mother, they found something unexpected – an “excess” of children with mothers born in the same month.

Overall, the researchers said, there were 4.6% more births than expected where the mother and child shared the same birth month.

Data also showed there were 12.1% more births than expected where siblings had birthdays in the same month and 2% more births than expected where the father and child shared the same month of birth.

Parents born in the same month as one another were found to occur more often than predicted.

The researchers said their work is the first to uncover the “tendency” of members of the same family to be born in the same season of the year, which also extends across generations.

They said that the time of the year a person is born may be related to family dynamics rather than being just a random variable.

Dr Adela Recio Alcaide, an epidemiologist at the University of Alcala, said: “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.

“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.”

For the study, published in the journal Population Studies, the researchers looked at all births in Spain from 1980 to 1983 and from 2016 to 2019 and all births in France from 2000 to 2003 and from 2010 to 2013.

The records provided the child’s month of birth, as well as those of their parents and siblings.

The researchers found there was a “spike” in January births amongst mothers who themselves were born in January and the trend was also seen across other months of the year.

The team said further research is needed to confirm the results and understand more about the implications.

The researchers wrote: “Rather than being a random variable, birth season seems to be related to family characteristics, which should be controlled for when assessing birth-month effects on subsequent outcomes.

“This study has added evidence on the dynamics underlying birth seasonality by unravelling a similarity that can be uncovered only through the use of large microdata sets.”