In almost every entourage, there is this family with three or four children, all of the same sex. Some see it as an astonishing coincidence, others speak of “girl genes” or “family of boys”. Behind these stories of family meals lies a real scientific question, which has long remained without a clear answer.
A team from the Harvard TH Chan School of Public Health has just published in the journal Science Advances a study that challenges the idea of simple chance. By analyzing 146,000 pregnancies of 58,007 American nurses between 1956 and 2015, researchers sought to understand why some parents only have girls or only boys.
Why do some families only have girls or boys?
On a population scale, the sex of a baby should look like a near-perfect coin toss: the sperm that fertilizes the egg carries either an X or a Y chromosome, and on average 105 boys are born for every 100 girls. However, the team did not look at each birth in isolation, but at complete siblings, family by family.
They showed that three successive boys gave a 61% probability of having a fourth boy, and that three girls in a row was accompanied by a 58% risk of welcoming another girl. “If you’ve had two or three girls and are trying for a boy, know that your chances are not 50-50. You are more likely to have another girl“, summarized Jorge Chavarro, professor of nutrition and epidemiology and lead author of the study, in the Washington Post.
The age of the mother, a game-changing factor
Among the eight maternal characteristics scrutinized, such as height, BMI or blood group, only one stands out: the age of first pregnancy. Women who became mothers after age 28 had a 43% chance of subsequently having only girls or only boys, compared to 34% among those who had their first baby before age 23. “Older age at first birth linked to greater chance of having children of the same sex“, explained Siwen Wang, first author of this work, in the journal New Scientist.
Why later first pregnancies matter
To explain this effect of age, the authors only put forward hypotheses. Over the years, the first phase of the menstrual cycle would shorten and the vaginal pH would become more acidic, two parameters likely to favor Y or
Genetic clues from the mothers
The authors also sequenced the DNA of 7,530 participants and found two variants associated with unisex siblings: the NSUN6 gene, linked to female offspring, and TSHZ1, associated with male families. “We were able to identify clear trends just from the mothers’ DNA. But we still lack paternal data to understand how the genetic factors of both parents interact. This is an essential avenue for the future“, underlined Siwen Wang, recalling that the effect remains modest and that these results will have to be confirmed in other populations.