Genes greatly influence when and how many babies a woman will have, study finds

Genes play a significant role in determining when modern women have their first baby and how many children they have in their lifetime, a study has found.

The findings suggest that human fertility is still evolving despite the fact that most women in Britain and other developed countries override their genetic instincts by delaying the start of their family.

Oxford University researchers said their study, based on analysing the genomes of thousands of women, shows that about 15 per cent of the difference in modern women in the age of first birth, and 10 per cent of the difference in total number of children, is due to genes.

But while there is a significant genetic predisposition that influences when a woman decides to start having a family, most women in Britain and other western countries are continuing to delay the timing of their first pregnancy, they said.

Women in Britain are now having their first baby about 4-5 years later than in the 1970s, with an average age now of between 28 and 29 years old when they first become mothers, the researchers said.

This delay was explained by social factors entirely, such as the number of women entering university or the labour market and the widespread availability of contraception.

However, the researchers suggest that there could be a biological explanation to explain at least some of the variation in the timing of a woman’s first baby and the overall size of her family.

“In evolutionary and genetic terms, this suggests that younger generations today should be inclined to have children at an earlier age than women in the past,” said Professor Melinda Mills, who led the study at Oxford published in the on-line journal Plos One.

“However, what we actually observe is that the reverse is happening. Social and environmental factors mean women in modern societies are delaying starting families, knowing that there is the risk of becoming infertile if they leave it too late,” Professor Mills said.

“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,” she said.

The study analysed the smallest genetic differences between 4,300 unrelated women in The Netherlands, and those between 2,400 female twins in the UK, and compared these inherited traits with the age they were when they first gave birth, and how many children in total they have had.

Unlike previous studies of this sort that concentrated on twins, the research was able to look at unrelated women to separate out further the influence of genes on fertility from the effects of upbringing and environment – such as whether someone goes to university.

“We found that the differences in women’s age at first birth and the number of children ever born were associated with genetic differences. For the age of first birth, 15 per cent of the observed variance was explained by genetic variation in common genes, for the number of children it was 10 per cent,” Professor Mills said.

There was an overlap in the genetic influence on these two aspects of fertility which suggests that the same genes that are related to when women have their first child are also involved in influencing the number of children they finally have, she said.

Felix Tropf of the University of Groningen in the Netherlands, the first author of the study, said: “In the second half of the 20th Century, women across many societies delayed starting a family. 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.”