A new study published in PLOS ONE suggests that genetics has a say in the timing women nowadays have their first babies. According to the results of the research, evolution has shifted fertility of women to a younger age, even if women nowadays still choose to have children at a later age.
The researchers of the new study aimed at finding why are women having babies 4 to 5 years later in our contemporary societies than women in 1970. The average age of women when they have their first child nowadays is between 28 and 29.
6,700 women from the UK and the Netherlands took part in the study and they were categorised in two groups: 4,300 of them were unrelated women and 2,400 of them were female twins.
When the DNA of the women was analysed by the scientists, they found that a set of genes impacted upon the age women have their first child and the number of children they have. They discovered that genetics influence about 15 % of the variation in the age the women’s first child is born, and 10 % of the difference in the number of children they have.
“What we see in this study is a clear genetic component linked to the age of mothers when they have their first child, and to the number of children they have,” said lead author Melinda Mills from Oxford University in a statement.
The two sets of genes also demonstrated an overlapping: it was more likely for those women having their first child at an earlier age to have more children eventually. The genes expressing early fertility would thus be inherited more often.
“Natural selection is not just a historical process. Modern societies are still evolving today, with early fertility patterns being an inherited reproductive advantage,” explained the authors.
However, in spite of what our genetics are inclined to, the opposite is happening in our modern world: women are still having babies at a relatively older age. The team of researchers interpret this apparent discrepancy in terms of factors in our society that work to cause the delay.
“Although genes play a significant part, it seems wider social changes, such as an expansion of women in further education and work, as well as the availability of effective contraception, are having a stronger effect on determining when women in modern societies have children,” said co-author Felix Tropf from the University of Groningen in the Netherlands.
In the future, the study might perhaps help women to have their DNA tested to find out for how long can they wait before having a child. They could also identify the optimum age at which they could start trying conceiving.
“This research tells us there are genetic differences between women which could be significant for women making decisions about when to have their first baby,” said Mills.