TORONTO — The number of times a person has given birth may affect how quickly they physically age, according to a new study.
Researchers from Pennsylvania State University in the U.S. found that people who have given birth three or four times appeared to be biologically aging more slowly than those who have given birth fewer than three times or more than four times.
The findings, published in the journal Scientific Reports last week, also showed, however, that these effects were only evident after a person had gone through menopause.
“Our findings suggest that pregnancy and birth may contribute to the changing and dysregulation of several different physiological systems that may affect aging once a person is post-menopause,” Talia Shirazi, one of the study’s authors and a doctoral candidate in biological anthropology at Penn State, said in a press release.
“This is consistent with the metabolic, immunological, and endocrinological changes that occur in the body during pregnancy and lactation, as well as the various disease risks that are associated with pregnancy and reproductive investment more generally.”
For the study, the researchers examined data on 4,418 participants from the U.S. Centers for Disease Control and Prevention’s National Health and Nutrition Examination Survey, which was collected between 1999 and 2010.
The data included information on participants’ reproductive health, the number of live births they had, and if they had gone through menopause. The researchers acknowledged, however, they didn’t have access to data on miscarriages or aborted pregnancies, which could also be associated with the costs of reproduction – one of the study’s limitations.
The study’s authors then studied the reproductive data to measure biological aging by looking at nine biomarkers that are designed to assess metabolic health, kidney and liver function, anemia and red blood cell disorders, and immune function and inflammation.
Waylon Hastings, a post-doctoral researcher at Penn State who worked on the study, said they wanted to analyze markers that would indicate the age and functioning of the body’s major organ systems, instead of just looking at age on a cellular level.
“When we think about pregnancy, we don’t think about changes to individual cells but instead about how the immune system or metabolism changes, for example,” he said.
When they did this, they found a “U-shaped relationship” between the number of births a person had and accelerated biological aging.
Participants who reported never giving birth or giving birth fewer than three times and those who had given birth more than four times appeared to have quicker biological aging than those who had three or four births, the researchers said.
The study’s authors said this was even true when they controlled for other factors, such as chronological age, lifestyle, and other health‐related and demographic indicators.
According to the authors, pregnancy and breastfeeding use a large amount of the body’s energy and can affect its systems, including immune function, metabolism, and blood pressure.
“We think there’s something going on, some sort of trade-off, between aging and reproduction,” Shirazi said.
“This makes sense from an evolutionary biology point of view, because if you’re spending energy in pregnancy and breastfeeding, you probably don’t have as much energy to allocate towards things like physiological maintenance and defence.”
Shirazi said one possible explanation for the findings is the presence or lack of presence of ovarian hormones, which are protective against accelerated aging, in post-menopausal people.
“It’s possible that in pre-menopausal women the effect of hormones are buffering the potential negative effect of pregnancy and reproduction on biological age acceleration. And then perhaps when the hormones are gone, the effects can show themselves,” she explained.
While the researchers said their study shows an association between the number of live births and biological aging, it doesn’t show causation because the participants’ data was taken at only one point in time in their lives.
The research was also limited by the fact the data was collected only in the U.S. and they were unable to compare them with samples from non-Western, educated, industrialized, rich, and democratic (WEIRD) countries.
“WEIRD and non-WEIRD countries are characterized by significantly different activity patterns, nutrition, infectious disease ecology, and morbidity and mortality, all of which could shape costs of reproduction,” the study said.
In light of these limitations, the study’s authors said additional research is needed to better understand the connection between aging and having children.
“This transition into menopause, and female reproductive health in general, is very much under researched and not as well understood as it should be at this time,” Hastings said.
“If we can see that there are these changes in aging as a function of reproduction and menopause, and we don’t have a great explanation for why, then that’s a sign we should investigate this more.”