A new study has shown something most mothers have suspected for centuries – each additional child literally ages a mother's cells beyond their years.

In fact, cellular aging was accelerated by between 0.5 and 2 years for each extra baby, a more extreme effect than the scientists were expecting.

But, interestingly, the research also revealed a more surprising paradox – while pregnant, the mothers' cells actually appeared remarkably youthful.

The biochemistry behind the reproduction's aging tax helps explain why mothers tend to be more greatly affected by illness and live shorter lives than other women. It also gives mothers a new way to guilt trip their recalcitrant teenagers. Win!

Research led by Northwestern University in Illinois has added more detail to the growing pile of evidence that says child bearing carries a heavy biological cost to the mother right down to a cellular level, while also finding this aging process reverses during the pregnancy itself.

The researchers looked specifically at stretches of DNA at the ends of chromosomes called telomeres. As cells copy themselves, each generation of chromosome shortens as it loses a portion of this code.

They also analysed the prevalence of chemical tags that are clipped onto certain sequences, modifying their transcription.

Many of these so-called epigenetic markers are also closely related to the aging process, though not as a result of cells making copies in the same way as telomere shortening.

"Telomere length and epigenetic age are cellular markers that independently predict mortality," says the study's lead author, anthropologist Calen Ryan.

"Both appeared 'older' in women who had more pregnancies in their reproductive histories."

This isn't the first study to make such a discovery. Earlier this year, researchers from George Mason University found telomeres could shorten the equivalent of more than a decade among US women with one or more children.

This time another group of researchers analysed blood samples taken from more than 800 young women in the Philippines taken between 2005 and 2009, 40 percent of whom had given birth to between one to five children.

Once again there was a relationship between the number of children each woman had and the relative age of their biochemistry.

To add weight to the argument that each additional child was responsible for the clipped telomeres and epigenetic additions, the team also looked for signs of a reversed causation.

They found that having shorter telomeres isn't at all useful for predicting how many kids you'll have in the future, making it even more likely that it's those babies that are responsible for aging your cellular machinery.

The impact of big families might not be all that surprising for mothers in the Philippines, where a life of hard work wrangling a brood could conceivably contribute to an earlier senescence, especially compared with women in the western world.

A comparison of the DNA results with the women's socioeconomic status also failed to show any patterns, meaning this was less about how hard you work having more kids, and more about the impact gestation has on the body.

"We also didn't know if we would even detect such effects among the relatively young women in this population, all of whom were 20 to 22 years old at the time," says Ryan.

There was yet another surprise to come. Blood samples taken from women while they were actually pregnant revealed the complete opposite.

"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," says fellow researcher Christopher Kuzawa.

This temporary fountain of youth could be the result of all kinds of physiological, hormonal, and immunological adjustments the mother's body makes to accommodate a growing child.

Once the baby has been born, those changes are no longer necessary and the biochemical tide rushes back in, aging the cells once again. And then some.

The mix of telomere shortening and epigenetic edits suggest at two completely separate biochemical processes – one affecting the way cells reproduce, the other as a result of ongoing genetic processes.

Exactly which gestational processes play a key role in the process is a question yet to be answered. There's also speculation over whether those changes are truly permanent.

The team are already looking at more longitudinal studies.

"We still have a lot of questions to address that we hope will help us understand how factors like socioeconomic status and diet might contribute to costs of reproduction in women," says Ryan.

Not that we're saying you need to wait before telling your kids they literally made your telomeres shorter, so show some gratitude!

This research was published in Scientific Reports.