Why do some mothers have more boys than girls?

Why do some women only have girls or only boys? This is a question many people ask when they observe that some families with three or four children all have the same biological sex. Is there a genetic factor that determines this? In human populations as a whole, if no external discriminatory factor is at play—such as sex selection at birth or wars, among others— The ratio of biological male to female sex is very close to 1:1Certainly, there is a slight increase in the percentage of boys compared to girls among newborns, but after one year of life, these percentages are already equivalent. The fact that humans have few offspring makes it difficult to analyze whether there is any genetic factor skewing the proportion when we are only looking at the progeny of a single couple. With two or three children, statistically it is difficult to distinguish the action of chance from the action of some genetic or environmental factor, which must be subtle.

In principle, considering the determination of biological sex in the human species –determined by the presence of the SRY gene on the Y chromosomeIt has always been thought that the probability of having a son or a daughter was 50%, since when males produce sperm, half carry an X chromosome and the other half a Y chromosome. But is the probability of conceiving a boy or a girl really like flipping a coin? A 2020 study examined the Swedish population of people born from 1932 onward who had children before 2014. The analysis of this group of 3.5 million individuals, with a progeny of almost 4.8 million people, goes to conclude that there were no signs of a consistent deviation in the biological sex of offspring in different families

Although the study was conducted with a very large number of people, the analysis did not differentiate between genetic contributions specifically for each sex in detail and, furthermore, has a conceptual limitation, since the analysis to demonstrate the existence of genetic bias only assumed that if there were genetic variants, only variants would be transmitted. This would imply that siblings with the same parents would be more likely to have offspring of the same sex, with an almost Mendelian inheritance pattern or few genes involved, when it is very likely that there is a contribution from many individually minor genetic variants, to which must be added the influence of poorly defined environmental factors.

Women predisposed to having children of only one sex

However, a recent study using data from a cohort of 58,000 women who had more than two children between 1956 and 2015, collected by midwives in the United States, takes a slightly different approach, attempting to determine the probability of a single mother having children of only one sex. The data from this study would indicate that it is possible. There is a genetic component in some women that would predispose them to have children of only one biological sex.as if the coin tossed in the air weighed slightly more on one side than the other.

One of the first results they find is that when they look at the data for all births and focus on women who have only had two children, the girl-to-boy ratio is almost 1:1, which could indicate either chance or, very likely according to sociological data, that many couples prefer to have one child of "the pair" and decide not to have more offspring. In human reproduction, especially since methods for regulating offspring exist, desire and family projections play a very important role: many couples who have several children of the same biological sex may seek a new child of the opposite sex. To avoid this effect, the authors of the study also analyze the data without taking into account the sex of the last child, increasing the statistical significance. That is to say, there are mothers who have more sons and mothers who have more daughters in a non-random way. According to their data, mothers who had previously had three boys had a 61% probability of having a fourth boy, and for mothers who had already had three girls, the probability of having a fourth was 58%, values clearly deviating from the expected 50%.

Another interesting result, which would indicate the action of environmental factors, is that women who had their first child feel Women over 28 years of age had a 13% higher probability of having children of only one biological sex compared to younger mothers.In general, older mothers have shorter follicular cycles and a more acidic vaginal pH, but while a short follicular cycle would favor sperm with a Y chromosome (therefore, a boy), an acidic pH would favor sperm with an X chromosome (a girl). Each woman over 28 years of age could have a different predisposition to these factors that potentially contribute to this bias, but these are still hypotheses.

In conclusion, we still don't know why some mothers have more sons than daughters, although population studies show that there are mothers with a statistically significant deviation from the 1:1 ratio. However, the fact that couples currently have a very low number of children may limit interest in continuing similar studies. Furthermore, although at present in the European Union sex selection of embryos is only permitted in cases of serious rare diseases linked to the X chromosome, in other countries there are assisted reproduction clinics that offer parents the option to choose the biological sex of their future offspring, so this issue may be irrelevant.