Childbirth inevitably results in significant changes to a woman's mind and body, but the ultimate lifelong consequences of motherhood could be far greater than we ever realised.
An analysis of DNA collected from nearly 2,000 reproductive-age women in the US reveals that those who had given birth showed evidence of altered genetic markers suggesting they'd undergone significantly accelerated cellular ageing.
"We were surprised to find such a striking result," epidemiologist Anna Pollack from George Mason University told New Scientist.
"It is equivalent to around 11 years of accelerated cellular ageing."
Pollack and her team analysed data from the National Health and Nutrition Examination Survey (NHANES) – a broad cross-sectional study charting the wellness of people in the US over time.
When they examined data from the period 1999–2002 – years in which the survey included measurements of a genetic marker called telomeres – they noticed something unusual.
Telomeres are molecular regions that act as caps on the ends of our chromosomes, helping to protect the genetic information in our cells from deteriorating over time – and, hypothetically, from exposure to things that are harmful to our health.
In that vein, telomere length is taken as a marker of how old we are on a cellular level, with longer telomeres considered better to have, since shorter telomeres have been associated with outcomes like cancer, heart disease, and cognitive decline.
Now, we might have something new to add to that list: childbirth.
In the study, the team found that once they'd adjusted for things like age, ethnicity, education, smoking status, and so on, 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.
This average meant an adjusted difference of 116 fewer base pairs in women who had given birth, which the researchers explain is equivalent to around 11 years of accelerated cellular ageing.
What's amazing is that this telomere shortening associated with childbirth is even greater than what's previously been observed in research examining the association seen with smoking (a cost of 4.6 years of cellular ageing) and obesity (8.8 years).
What's more, in the study, the telomere shortening seemed to vary depending on how many children the women had delivered.
"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.
It's worth bearing in mind that due to the observational nature of the study, we can't conclude a causation effect here, only a correlation.
And at least one study has produced a contradictory result, with a 2016 study of rural Kaqchikel Mayan communities in Guatemala finding that women in the community with more surviving children had longer telomeres, suggesting that having children could actually protect women from cellular ageing.
Other previous studies have also quantified the size of telomere base pair reductions with less advanced terms of cellular ageing, which the researchers say could mean childbirth is only associated with about 4.5 years of advanced ageing.
At least one researcher not involved with the study has suggested the effect might be as little as three years of biological ageing.
As for what could be behind the telomere shortening seen in the US sample, the researchers speculate stress involved in looking after children could be involved, but given how little research has been conducted in this area, they advise their results should be treated with caution.
"We're not saying 'don't have children'," Pollack told New Scientist, and while scientists keep examining what's really going on here, that's very level-headed advice.
The findings are reported in Human Reproduction.
A version of this article was first published in February 2018.