We all know that pregnancy and childbirth change women’s minds and bodies. A new study has found that women who give birth can age very fast, genetically speaking. But how?

Researchers collected DNA data from 1,505 different women from the US, with ages ranging from twenty to forty-four and discovered that having children significantly altered genetic markers of aging — telomeres, to be exact.

Telomeres are repetitive DNA fragments found at each end of the chromosomes, which protects them from deterioration or from fusion with neighboring chromosomes. At birth, our telomeres are long, but with each cell replication, telomeres grow shorter. Thus, telomere length decreases from birth to death and is considered a marker of aging. Shorter telomeres are correlated with outcomes like cancer, heart disease, and cognitive decline. Another cause of telomere shortening is stress,

Epidemiologist Anna Pollack from George Mason University and her team analyzed data from the National Health and Nutrition Examination Survey (NHANES) – one of the largest cross-sectional studies charting the wellness of people in the US.

Researchers analyzed data collected between the years 1999–2002, a period in which the survey included telomere measurements, and discovered something unsettling.

Once the team had adjusted for things like age, ethnicity, education, and smoking status, they discovered that women who had given birth to at least one child had telomeres that were 4.2 percent shorter on average than those of women who had not borne children.

Researchers explain that this percentage translates to around 11 years of rapid cellular aging. Compared to smoking (a cost of 4.6 years of cellular aging) and obesity (8.8 years), motherhood seems to be the champion of accelerated  DNA aging.

The study also revealed that the more children you have, the more your telomeres shrink.

“We found that women who had five or more children had even shorter telomeres compared to those who had none, and relatively shorter relative to those who had one, two, three or four, even,” Pollack told Newsweek.

The authors attributed telomere shortening to the stress accompanying having children, but they are not yet entirely sure of the cause. This study was purely observational, showing only a correlation between the two.

A 2016 study that analyzed telomere size in Mayan communities in Guatemala found that women in the community that had more surviving children had longer telomeres, suggesting that having children could actually protect women from cellular aging. Researchers believe that Mayan communities give more social support to their mothers than the US does — a great deal of stress being involved in the upbringing of the US kids.

“Anecdotally, just chatting with my friends who have children, we all do feel that having kids has aged us,” Pollack said to Newsweek. “But scientifically, this does fit with what we understand pretty well. We know that having kids is associated with a higher risk of heart disease and diabetes. And some large studies have linked telomere length to mortality risk and risks of other major diseases.”

Of course, having a child doesn’t mean you literally age 11 years. The authors write that their dataset lacked information on social factors, stress and fertility status, which may help explain these findings. With only two other previous studies regarding this matter being published, this paper‘s findings should be interpreted with caution, the authors warn.

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