Will it be male or female? The mother’s age to the first pregnancy and genetics influence the probability of having all the children of the same sex.
It is not the shape of the belly and not even the pure chance that decides whether the birth bow will be pink or blue: the determination of the sex of the unborn child is not entirely random as it was believed, but it looks more like the launch of an unbalanced coin, in which head or cross have a greater advantage of going out.
The chances of each family with more children to find themselves with only male children or only females seems be influenced by maternal genetics, and from the age of the mother to the first pregnancy: it is not equal to that “50 and 50” that is used to mention before ultrasounds and baby showers.
Genetics and age: the analysis on over 146,000 pregnancies
It is used to think that, at every birth, the probability for a couple to put a male child or daughter in the world are given to 50 and 50. The experience, however, shows us that some families have more children of the same sex and that for some couples, despite the number of pregnancies, the long -awaited pink (or blue) bow seems to never appear.
A group of scientists from the Harvard that School of Public Health analyzed the maternal age and the genomic data of 58,007 women with a story of at least two single parts behind them, For a total of 146,065 pregnancies From 1956 to 2015. A clear limit of the study is the extreme homogeneity of the sample, made for 95% of white women, residing in the United States and a nurse profession.
The researchers tried to understand if genetic or personal factors (the mother’s age at the first pregnancy) had had a weight in determining the sex of the unborn child. For greater precision, the information on the sex of the latest born of each woman have excluded from the data, to avoid including in analysis dictated behaviors – for example, by the decision to stop having children after the male, or the desired female, or after a balance between males and females in the family had arrived.
A coin launch? Yes, but … made up
Both maternal age and genetics They influenced the probability of bringing the same sex to the world of the same sex: they are both factors that deviate the basic probability, moving it from the hypothetical center. A higher maternal age at birth It was correlated with higher probability of having only males or only females; The expression in the maternal DNA of the NSUN6 gene, on the other hand, corresponded to a greater probability of having only females, that of the TSHZ1 gene, of having only male children.
On birth sex then probably affect paternal factors not considered in the study.
Therefore, when women themselves (genetics, age) are taken into consideration instead of their pregnancies, the determination of sex at birth It is not completely random.
“Both biological factors and reproductive decisions of delaying pregnancy They can contribute to the observed phenotype of grouping by sex within families », write the authors of the study. And while trying to understand to what extent genetic and age weighs on the grouping by sex within families, those couples “who want to have children of more than one sex and who have already had two or three children of the same sex must be aware that, when they try to have a subsequent son, they are probably launching a two -headed coin”.