Cellular ageing was accelerated by between 0.5 and 2 years for each additional pregnancy.
Washington: Multiple pregnancies might make women's cells age more quickly, according to a study that may explains why women with many children tend to show signs of accelerated ageing.
The findings, published in the journal Scientific Reports, were reached by looking at two separate markers of cellular ageing - telomere length and epigenetic age - in hundreds of young women with different reproductive histories in the Philippines.
"Telomere length and epigenetic age are cellular markers that independently predict mortality, and both appeared 'older' in women who had more pregnancies in their reproductive histories," said Calen Ryan, a doctoral student at University of Washington in the US.
"Even after accounting for other factors that affect cellular ageing, the number of pregnancies still came out on top," said Ryan.
Cellular ageing was accelerated by between 0.5 and 2 years for each additional pregnancy, a surprisingly large effect according to the researchers.
Another finding they did not expect was the fact that women who were currently pregnant had cells that looked younger - not older - than predicted.
"Paradoxically, even though a woman's biological age was higher with each child that she had, if a woman was pregnant when the measurements were taken, her epigenetic age, and to a lesser extent her telomeres, looked 'younger' than predicted for her chronological age," said Christopher Kuzawa of Northwestern University in the US.
"It's an interesting situation in which pregnancy makes someone look temporarily 'young,' but there appears to be some lasting, cumulative relationship between the number of pregnancies and more accelerated biological age," said Kuzawa.
Researchers have known from historical records and epidemiological studies that women who have many children tend to have slightly shorter lives and succumb to different diseases than those who do not.
"What we didn't know was whether we could detect these kinds of effects using measures of cellular aging," Ryan said.
Although there is good evidence that having more children, especially more than four or five, can increase the risk of certain diseases and shorten lifespan, researchers still do not really know why.
"Our study points to cellular changes during pregnancy, possibly related to adaptive changes in the mother's immune system as a possible explanation," said Kuzawa.
"There's still a lot we don't know. For instance, it's not clear whether these relationships will persist into later life as these women age. We also do not know whether these changes will actually lead to less favourable long-term health outcomes," he said.
To answer these questions, a follow-up study on the same women 13 years after the first measurements, taken in 2005, is already underway.