Do you and your mom share the same birth month? Scientists reveal the phenomena is common - and they know why

It is a strange phenomenon - mothers and children sharing the same birth month.

While the occurrence sounds like one in a million, a new study has found it happens more commonly than expected.

Researchers from Spain and the US analyzed 10 million births from 1980 to 1983, 2016 to 2019, 2000 to 2003 and from 2010 to 2013, finding there were 4.6 percent more births in which mother and child shared the same birth month than expected.

The team believes the results come down to socio-demographic characteristics - people of similar backgrounds are known to pair up and to be more likely to give birth at certain times of the year.

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 team looked at sociodemographic characteristics of mothers, such as maternal education, age, parity, re‐partnering, race, social class, birth order or legitimacy.

The team looked at all births in Spain from 1980 to 1983 and from 2016 to 2019 and all deliveries in France from 2000 to 2003 and 2010 to 2013. 

The records provided the child's month of birth, as well as those of their parents and the sibling that was closest to them in age.

Researchers shared that birth in a particular country tends to follow a pattern, with more babies being born at certain times of the year than at others- this is known in academic literature as birth seasonality.

However, the team wanted to see if mothers and children share the same birth month.

Data showed a spike in January births among the mothers born in January and the same was observed in February babies, and so on.

This was true for both countries and all four time periods studied.

The team also found similar cases among siblings - 12.1 percent more siblings' births in the same month than hypothesized. 

The study also found 4.4 percent more births with parents having the same birth month and two percent more with children and the father.

'In Spain, for example, a woman with a higher education is more likely to give birth in the spring than a woman without a higher education,' researchers noted.

'If she has a daughter, in addition to being more likely to be born in the spring, this daughter may be more likely to have higher education since her mother has it. 

'Thus, when this daughter has children, she will be more likely to have them in the spring too.'

These findings were likely due to allowing the daughter to experience the same higher education as her mother.

Factors that can affect the biology of fertility, such as availability of food and exposure to sunlight, can also vary according to a person's background.

'The excess of children with a father and mother born in the same month seems to be due to social or behavioral 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,' said Recio Alcaide.

'This,' adds co-author Professor Luisa Borrell, from The City University of New York, 'may not be surprising considering things such as partnerships tend to be formed by people with similar socio-demographic characteristics.'

'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,' Borrell added.