“One chance in two of having a girl or a boy!”, common sense assures every pregnancy. But this 50/50 probability does not hold true in all families, concludes a new study published in the journal Science Advances. If you have three children of the same sex, for example, the chance that the fourth will be the same sex is 60%.
Consider the family and not the individual as a unit
“ Families who want offspring of more than one sex and have already had two or three children of the same sex should be aware that in trying to have the next one, they are probably tossing a loaded coin.", conclude this new work on a database of more than 58,000 American nurses between 1956 and 2015, chosen for its quality.
Previous studies have looked at genetic factors affecting the sex of offspring at the individual level, but have found nothing conclusive. This individual-level approach assumes that all births are independent events", analyzes epidemiologist Siwen Wang, first author of this work. " By considering the family – and not the individual birth – as the unit of analysis, we are able to detect clustering and verify whether certain biological or genetic factors influence the sex of offspring at the family level.. »
Mother's age at first pregnancy matters
Two factors are particularly important for a mother regarding the sex of her future children: her age at the time of her first pregnancy and the presence of one or other of two genetic factors, each predisposing to the birth of one of the two sexes. Thus, a woman who has her first child after the age of 28 has a 13% higher probability of having children of the same sex than a woman under the age of 23. A difference that is certainly modest, but significant over a large population.
The biological reasons for this age influence are not known, but researchers have a hypothesis. With age, women experience physiological changes such as a shorter follicular phase and lower vaginal pH", explains Siwen Wang. However, the acidic (low) pH tends to favor the survival of the Y chromosome, which is larger and more resistant than the Y chromosome. " But at the individual level, these effects can differ from woman to woman, so aging can tip the scales in favor of one sex or the other, depending on their specific biology.", specifies the researcher.
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Genes associated with same-sex births
Two maternal genetic variants, near the NSUN6 and TSHZ1 genes, were also associated by the researchers with having only girls or only boys, respectively. The biological mechanism behind these actions is not known, but both are involved in regulating gene activity and therefore the production of certain proteins. These genes are important for fundamental biological processes and could, in theory, play a role in pregnancy-related functions." suggests Siwen Wang. " The discovery of these genome-wide significant maternal variants was particularly surprising." , as well as " strength and consistency » groupings by sex based on indicators, even when taking behaviors into account. The calculations took into account one of the main challenges anticipated by scientists: the bias of "collectors" who continue to have children in order to obtain the desired sex.
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“ What's fascinating is that we were able to detect significant patterns from maternal data alone, including genetic variants linked to having only boys or only girls", concludes Siwen Wang. " That said, not having information on biological fathers prevents us from fully exploring how maternal and paternal factors interact, which is an important direction for future research.. »
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