Having children ages women's DNA by 11 years, new research suggests.

Giving birth shortens women's telomeres by around 4.2 per cent, a study found.

Telomeres "cap" the end of DNA strands, with longer lengths being associated with slower aging, longer lifespans and improved overall health.

Such an extent of telomere shortening is greater than the effects of smoking or obesity demonstrated in previous studies.

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Study author Dr Anna Pollack from George Mason University, Virginia, said: "We were surprised to find such a striking result. It is equivalent to around 11 years of accelerated cellular ageing."

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Researchers believe this may be due to the stress of raising children, particularly in countries without mandatory maternity leave, such as the US.

They stress, however, more research into the link between motherhood and genetic ageing is required, with Dr Pollack adding: "We're not saying 'don't have children'."

How the research was carried out

The researchers from George Mason University, Virginia, analysed 1505 women aged between 20 and 44 who took part in the National Health and Nutrition Examination Survey between 1999 and 2002.

Of which, 444 had never given birth.

This survey was chosen because it measured its participants' telomere lengths via blood samples.

The study's participants' live birth histories were determined via a questionnaire.

The findings were published in the journal Human Reproduction.

"It's like adding three more drops to a teaspoon"

The results contradict a study carried out in Malaysian women, which suggested having more children lengthens telomeres.

According to the more recent investigation's researchers, this may be due to the participants in the former study receiving greater social support.

Dr Carmen Martin-Ruiz, from Newcastle University, who was not involved in either trial, believes altered telomere length due to motherhood may only really add around three years to DNA ageing, saying, 'it's like adding three more drops to a teaspoon'.

Sex slows DNA ageing in women

This comes after research released in July last year suggests having sex at least once a week slows ageing in women, even if they do not enjoy being intimate.

Being active between the sheets increases the length of women's telomeres, a study found.

Women's telomeres lengthen with regular love making regardless of whether they are sexually satisfied in their relationship, the research adds.

Researchers from the University of California in San Francisco believe sex may aid ageing in women by dampening stress and boosting their immune systems.