Parenting pregnancy

Could Genetics Play a Role in the Timing and Size of Families?

There are plenty of factors that might play into when a woman decides to start a family and how large that family will be, but could one of those factors be genetics? After studying the genomes of thousands of women from the UK and Netherlands, researchers from the University of Oxford believe that answer to be yes . . . but only to an extent.


Published in the journal PLOS ONE, the study used the latest advances in molecular and quantitative genetics to extensively examine the data on genomes from 4,300 unrelated women in the Netherlands from the Lifelines Cohort Study and 2,400 women from the TwinsUK cohort (one twin randomly selected). The two groups were pooled together to create a pool to analyze for how much of a role genetics might play in the age in which women started families and the size of those families.

According to the researchers, genes were found to account for about 15 percent of the differences between modern women on when they had their first baby, and 10 percent of the differences in the number of children they had. They also discovered an overlap between these two genetic effects, which may explain, in part, why women who have children earlier also tend to have more children.

Previous studies, which were smaller in scale, relied on datasets of twins or those within families, but this study used unrelated women. And, because they combined the genetic results from both samples, they found that natural selection is not just a historical process. But modern societies are still evolving, and even though early fertility patterns are being inherited, other factors are trumping the genetics behind it, to an extent.

“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,” project leader Professor Melinda Mills, from the Department of Sociology at the University of Oxford and Nuffield College, told Science Daily. “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. 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.”

“In the second half of the 20th century, women across many societies delayed starting a family,” lead author Felix Tropf, from the University of Groningen in the Netherlands, added. “Although genes play a significant part, it seems wider social changes, as well as the availability of effective contraception, are having a stronger effect on determining when women in modern societies have children.”

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About the author


Kate Givans is a wife and a mother of five—four sons (one with autism) and a daughter. She’s an advocate for breastfeeding, women’s rights, against domestic violence, and equality for all. When not writing—be it creating her next romance novel or here on Growing Your Baby—Kate can be found discussing humanitarian issues, animal rights, eco-awareness, food, parenting, and her favorite books and shows on Twitter or Facebook. Laundry is the bane of her existence, but armed with a cup of coffee, she sometimes she gets it done.

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