Science finally pierces the mystery of couples having only girls (or boys), and that has nothing to do with chance

Science – How is it that families with three, four children, are only made up of girls, or only boys? Nevertheless, What determines the sex of an unborn child? Meanwhile, Chance, or biological predispositions? However, This is what researchers from the Harvard Th Chan School of Public Health, the Harvard University of Harvard.

In a study recently published in the journal Science Advancesthey demonstrate that sex at science finally pierces mystery couples birth is associated with the age of the mother. Therefore, with certain genes, but also with the sex of older brothers and sisters.

“If you have had two. For example, three girls and try to have a boy, know that your chances are not 50-50. Nevertheless, You are more likely to have another girl ”told the Washington Post Jorge Chavarro. Meanwhile, professor of nutrition and epidemiology and principal author of the study.

A percentage specific to each family – Science finally pierces mystery couples

To achieve this conclusion. the researchers analyzed data concerning more than 146,000 pregnancies of 58,000 nurses living in the United States between 1956 and 2015. “ This individual approach assumes that all births are independent events. explains the epidemiologist Siwen Wang, the first author of this work, and quoted by Sciences and future. Considering the family -. not individual birth – such as the analysis unit, science finally pierces mystery couples we are able to detect groupings and check whether certain biological or genetic factors influence the sex of offspring to the family level. »

This hypothesis has proven to be convincing since the results showed that some families were more. likely to have children of the same sex. Thus, mothers having already had three boys had a probability of 61 % higher to have one fourth. In the case of families with three girls, the probability of having a fourth was 58 %.

The mother’s age at the time of pregnancy

The study has also looked at biological. factors influencing the sex of the child. Its authors have thus discovered that maternal age at the time of the first pregnancy played a key role. in the sex of children at birth. Women who started to have children after 28 years had 43 % chance science finally pierces mystery couples of having one of one sex. compared to 34 % chance among those who were under 23 at the time of their first maternity.

According to Jorge Chavarro, this could be due to biological changes in women with age. His colleague Siwen Wang cites several. established in previous works: first, a shortening with the age of the first phase of the menstrual cycle, which could promote the birth of boys. Cited by the review New Scientistit also argues that a decrease in vaginal pH with age could promote the survival of sperm containing the X chromosome. thus increasing the chances of having a daughter.

A genetic predisposition

The study also identified two genes. NSUN6 and TSHZ1, respectively linked to having only boys or only girls. “We do not know why these genes would be associated with sex at birth. but they are, which opens science finally pierces mystery couples up new questions”says Jorge Chavarro.

Next step, for researchers: analyze the way in which behavioral, environmental and biological factors influence the sex of a baby. But also understand the role played by the Father in determining birth sex. “The fact of not having information on the organic fathers prevents us from fully exploring the way in which maternal. paternal factors interact, which is an important direction for future research”, Reconnanîm seven wang.