It is assumed that the probability of a woman having a child or daughter is 50/50, similar to the launch of an air coin. However, some women may be more likely to have children of one sex, depending on… age.
The proportion of boys and girls at birth is about 50/50 at the population, mainly because the sperm of men is as likely to contain a chromosome X or Y, which determines the sex of the child.
However, a new study has revealed that some women may be more likely to have children of one sex, depending on age.
On the sidelines of this study this Friday in Science Advancesdata from more than 58,000 mothers in the US were collected who had participated in two separate studies on contraception and maternal health.
Of these women, 61% had two children, 30% had three children, 8% had four children and the rest had five or more children.
As it details, the investigators compared the sex of women’s children with eight characteristics: height, body mass index, race, blood color, chronotype (the time of the day they are more alert), age of the first menstruation and the age at which they had their first child, which varied between 13 and 48 years old.
Or just boys or just girls
The team found that the most advanced maternal age in the first birth was associated with a more likely to have only girls or boys.
Women who were over 28 when they first gave birth had 43% hypotheses that they had a single -sex children later, compared to 34% of hypotheses among those under 23 when they were mothers for the first time.
None of the other characteristics were related to the sex of women’s children.
Researchers theorize that, although it is not sure why the maternal age may have this effect, this may be due to biological changes In the body, which vary depending on the woman.
In New Scientist, Joshua Wildefrom the University of Oxford, who was not involved in the study, says that the results can also be explained by behavioral factors.
“It is possible for older women to be more likely to plan the size of their families, he says. This may mean that even if they want to have two children of different sexes, they may decide to stop after having two boys or two girls, while a younger woman may be more likely to continue to have a child of the other sex, with less concern about family size,” he says.
Other theories
Previous studies suggest, for example, that the first phase of the menstrual cycle becomes shorter with age, which may favor the birth of boys.
In turn, a decrease in vaginal pH with age may favor the survival of sperm with X chromosomes, leading to a greater likelihood of having a girl.
