Scientists believe they have discovered why so many mothers have the same birth month as their children.
Researchers from Spain and the United States analyzed 10 million births between 1980 and 2013 and found that the phenomenon was more common than previously thought.
In fact, there were 4.6 percent more births in which mother and child occurred in the same birth month than previous estimates suggested.
Even for siblings who had their birthday in the same month, the figure was 12.1 percent more than expected.
The team believes the results are due to sociodemographic characteristics and biological factors, including the occurrence of family characteristics such as education, income and age.
Factors that can influence the biology of fertility, such as food availability and sun exposure, can also determine when a woman gives birth.
Researchers from Spain and the United States analyzed 10 million births from 1980 to 1983, 2016 to 2019, 2000 to 2003 and 2010 to 2013 and found that there were 4.6 percent more births in which the mother and child had the same birth month than expected
“Rather than being a random variable, birth season appears to be related to family characteristics that should be considered when assessing the effects of birth month on later outcomes,” says the study published in the journal Population Studies.
While many believe that having the same birth month as a parent is important to family planning, new research shows that this is not always the case.
Dr. Adela Recio Alcaide, an epidemiologist at the University of Alcala, said: “What could be causing the higher probability of family members being born in the same season?”
“The possible explanations appear to be both social and biological.”
The team examined sociodemographic characteristics of mothers such as maternal education, age, parity, repartnering, race, social class, birth order or legitimacy.
The team examined all births in Spain from 1980 to 1983 and from 2016 to 2019, as well as all deliveries in France from 2000 to 2003 and 2010 to 2013.
The records included the child's birth month as well as the birth months of his parents and the sibling closest to him in age.
Researchers said births in a given country tend to follow a pattern, with more babies being born at certain times of the year than others – this is known in the literature as birth seasonality.
However, the team wanted to find out whether mothers and children had the same birth month.
The data showed an increase in January births among mothers born in January, and the same was seen among February babies, and so on.
The team believes the results are due to socio-demographic characteristics – it is known that people from similar backgrounds form couples at certain times of the year and are more likely to give birth
This applied to both countries and all four periods examined.
The team also found similar cases among siblings – 12.1 percent more sibling births in the same month than expected.
The study also found 4.4 percent more births to parents with the same birth month and two percent more to children and the father.
“In Spain, for example, a woman with higher education is more likely to give birth in the spring than a woman without higher education,” researchers found.
“If she has a daughter, not only is she more likely to be born in the spring, but that daughter is also more likely to have a higher education because her mother has it.”
“So if this daughter has children, it is more likely that she will have them in the spring.”
These results were likely due to the daughter being able to receive the same higher education as her mother.
Factors that can influence fertility biology, such as food availability and sun exposure, can also vary depending on a person's background.
“The excess of children whose father and mother were born in the same month appears to be due to social or behavioral causes before conception related to the choice of a partner born in the same month, as we estimate this excess using Marriage statistics have been observed.” “Spouses are more likely to mate with someone from the same month,” said Recio Alcaide.
“This,” adds co-author Professor Luisa Borrell of the City University of New York, “is perhaps not surprising given that partnerships are typically formed by people with similar socio-demographic characteristics.”
“In addition, biological factors known to influence birth seasonality – such as photoperiod exposure, temperature, humidity and food availability – also depend on sociodemographic characteristics, as different social groups are exposed to these biological factors to varying degrees,” said Borrell added.