Women who give birth may be biologically "older" than women who don't, a new study suggests.

For the study, the researchers analyzed information from 1,556 U.S. women ages 20 to 44 who took part in a national survey from 1999 to 2002, which involved giving blood samples.

The researchers looked at the genetic material inside the women's cells, specifically the length of their telomeres. These are caps on the ends of chromosomes that protect the chromosomes from damage. Telomeres naturally shorten as people age, but the structures don't shorten at the same rate in every person. The longer a person's telomeres are, the more times their cells could hypothetically still divide, research has shown. Thus, telomeres are considered a marker of biological age — that is, the age of a person's cells, rather than the individual's chronological age.

Women in the survey who said they'd given birth to at least one child had telomeres that were about 4 percent shorter, on average, than those of women who'd never given birth. The findings held even after the researchers took into account other factors that could affect telomere length, including the women's chronological age, body mass index and smoking habits.

These findings suggest that a "history of live birth may be associated with shorter telomeres," the researchers wrote in their abstract, which was presented this week at the meeting of the American Public Health Association in Denver. [9 Uncommon Conditions That Pregnancy May Bring]

The study was not designed to determine the reason behind the link, the researchers said. But one hypothesis is that having children increases stress levels, and high stress has been linked with shorter telomeres, the scientists said.

"It is possible that pregnancy, birth and child-rearing can induce chronic stress, leading to shorter telomere length perhaps through an inflammatory pathway," study researcher Anna Pollack, an assistant professor and environmental and reproductive epidemiologist at George Mason University, in Fairfax, Virginia, told Live Science. However, because the survey was conducted at a single point in time, the researchers cannot determine which came first in the women's lives — giving birth or having shorter telomeres, Pollack said. It's also possible that for some yet-unknown reason, women with shorter telomeres are more likely than women with longer ones to have children, Pollack said.

More studies are needed that follow women over time and measure the length of their telomeres before, during and after pregnancy, she said.

"It would be interesting to see how telomere length changes during pregnancy, after birth and during the child-rearing years, and how these changes compare to women who do not have children," Pollack said.

Future studies could also investigate the findings further, by including a measurement of women's levels of cortisol, a hormone linked to stress, said study researcher Kelsey Rivers, an undergraduate student at George Mason University majoring in global and community health, who presented the findings. Other studies could compare telomere length in women who have given birth with telomere length in those who adopt children, to see if the effect might be linked to parenting or giving birth, Rivers said.

The researchers are working on publishing the findings in a peer-reviewed scientific journal.

Original article on Live Science.