How Many Kids You’ll Have (and When) May Be Written in Your DNA

A lot of factors contribute to how many kids you have, and at what age you have your first baby: your relationship status, career, socioeconomic position, cultural surroundings, and personal choice, to name a few. But a new study suggests that genetics may also play a small but significant role in family size and timing. In some cases, researchers say, a woman’s DNA could even help predict whether she has any children at all.

The study, led by scientists at the University of Oxford, found 12 regions of the human genome that seem to have an influence on reproductive behavior. Two of those regions were already suspected to be involved in sexual activity, the authors wrote, but 10 had not yet been identified as such.

The same genetic variants linked to having children at a later age were associated with other characteristics reflecting sexual development, such as the age at which girls have their first period, women experience menopause, and boys’ voices change during puberty.

“For the first time, we now know where to find the DNA areas linked to reproductive behavior,” said lead author Melinda Mills, PhD, professor of sociology at Oxford’s Nuffield College, in a press release. “For example, we found that women with DNA variants for postponing parenthood also have bits of DNA code associated with later onset of menstruation and later menopause.”

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Mills says these findings could potentially allow physicians and fertility specialists to personalize their advice for potential parents. “One day it may be possible to use this information so doctors can answer the important question: ‘How late can you wait?’ based on the DNA variants,” she said.

Several of the variants identified in the study appear to be related to biological processes (such as the production of follicle-stimulating hormone in women and sperm development in men) or conditions (such as endometriosis and polycystic ovary syndrome) that can affect fertility. The authors say that learning more about these genetic factors might also help predict the effectiveness of procedures such as in-vitro fertilization—which can be expensive and invasive, and don't always work.

Of course, genetics don’t tell the whole story; anyone who’s made conscious decisions about whether or when to have children can attest to that. Researchers determined that, together, these variants influence only about 1% of the timing at which men and women have their first child, and only about 0.2% of the variability in number of children a person will have.

Those numbers are low, say the authors, but they are meaningful. In fact, in certain cases, those variants can affect a woman’s chances of remaining childless by up to 9%. (Their calculations did not find a similar effect for men.)

“It is important to put this into perspective,” Mills said, “as having a child still strongly depends on many social and environmental factors that will always play a bigger role in whether or when we have babies.”

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In a video posted online by the University of Oxford, Mills stressed that the study does not mean people are "hardwired" to have a certain number of children at a certain age.

“Rather, everyone has a certain probability or propensity to start having problems with fertility at a particular age,” she says. With more men and women waiting longer to have children, she says, genetics is an important factor to consider.

As the human genome continues to be studied, the researchers anticipate that it will eventually be possible to predict 10 to 20% of variability, at most, in family planning.

“You have to think about it like a big puzzle,” Mills said. “Predictors such as did you stay in education longer, did your mother work, how many siblings did you have—these variables, these perspectives, when they’re looked at alone are all one little piece of the puzzle. When we add in the genetic aspect, we increase the overall explanation, and we almost finish the puzzle.”

The study is co-authored by more than 250 sociologists, biologists, and geneticists from institutions around the world, and published today in Nature Genetics. It combined data from 62 previous studies involving more than 340,000 people.

Nicola Barban, PhD, first author on the paper and senior research associate in sociology at Oxford, summarized the findings in this way: “Our genes do not determine our behaviour, but for the first time, we have identified parts of the DNA code that influence it.”