All things being equal, you’d expect there to be about a one in twelve chance of sharing your birthday month with someone else, right? Well, not quite – at least, not if that person is your mom.
“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,” reads a new paper from researchers at Universidad de Alcala and City University of New York.
“Women are more likely to have children in the same month as their own birth,” the team reports. “Moreover, the correspondence in month of birth extended to other family members: between fathers and children […] between adjacent siblings […] and between parents.”
The project analyzed data from more than ten million births in total. They used official records covering Spain from 1980 to 1983 and 2016 to 2019, as well as France from 2000 to 2003 and from 2010 to 2013. Importantly, these records didn’t only contain information about the new arrivals’ dates of birth – they also included the same for the parents and closest sibling.
The team was fully expecting to find more babies born at certain times of the year than others – this “birth seasonality”, as it’s known in academic circles, is a well-known phenomenon. For example, in English-speaking countries, you’re up to 40 percent less likely to be born on Christmas Day than on some random day in September.
There are a few suggested reasons for this, both biological and behavioral: “Hypotheses include deterioration of sperm quality during summer, seasonal differences in anterior pituitary-ovarian function caused by changes in the daylight length, and variation in quality of the ovum or endometrial receptivity,” explains a 2001 paper in the journal Obstetrics and Gynecology.
“Increased sexual activity associated with end-of-year holiday festivities has also been postulated as a possible behavioral explanation for the December peak in conceptions,” the paper adds, although “the exact reasons remain unknown.”
But what came as a surprise to the researchers working on this more recent paper was what happened when they split the data up according to mothers’ birth month – when those general birth seasonality trends disappeared. Instead, babies whose mothers were born in January, say, were 4.6 percent more likely than expected to also be born in January; the same was true for babies of February-born mothers; and so on throughout the calendar.
The effect was even more pronounced when comparing siblings: the rate of births within the same month as an older sibling was more than 12 percent higher than would be expected. Children were also very slightly more likely to share a birth month with their father – and, as logic would dictate given all that, there was also a higher chance of parents sharing a birth month.
That’s interesting enough, but the real question is, of course, why? The team has their suggestions – and they may not be what you’re expecting.
“The potential explanations seem to be both social and biological,” said Adela Recio Alcaide, an epidemiologist at the University of Alcala and lead author of the study, in a statement.
“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,” she explained. “We have observed this excess with marriage statistics, with spouses being more likely to mate with someone from the same month.”
So, the two-parents-one-month thing checks out. It’s actually not that surprising: “partnerships tend to be formed by people with similar socio-demographic characteristics,” the paper points out, and "evidence has shown on numerous occasions that different socio-demographic groups display different seasonal birth patterns.”
But what’s behind the apparent tendency for your mom to get down and dirty approximately three months after her birthday? Well, remember all those suggestions as to what’s behind birth seasonality more generally? Those tend to be socially specific, the team realized – and so, what mom experiences, so too will baby.
“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,” Luisa Borrell, a social epidemiologist at the City University’s Department of Epidemiology & Biostatistics, Graduate School of Public Health & Health Policy, said. “Different social groups are exposed to these biological factors to varying degrees.”
The paper is published in the journal Population Studies.