Genes may play a role in the age at which a woman has children, researchers say.
Genes also contribute to the number of women have, according to researchers from Oxford University, UK.
Researchers concluded that genes are 15% responsible for the age at which women give birth – and 10% responsible for the number of children they have.
The researchers stress that modern lifestyles are increasingly a key factor in women’s decisions about family.
But the findings, reported in the journal PLoS One, could reflect genetic contributions to hormone levels.
The findings come from a study of 2,400 female twins in the UK and 4,300 women in the Netherlands.
Researcher Professor Melinda Mills said: "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. 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."
She added: "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."
Human Fertility, molecular genetics, and natural selection in modern societies. PLoS ONE 3 June 2015 [abstract]
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