A new study suggests that the best age to give birth may evolve over long periods of time, the Daily Telegraph reported.

Of those who wait to conceive, only women genetically predisposed to longer lasting fertility will be successful. But, those women will then pass that gene on to their children, which will extend the average length of fertility, according to a study from the University of Sheffield, in the UK.

"The natural selection maintaining young-age fertility might weaken and the relative strength of natural selection on old-age fertility could increase, something that could potentially lead to improvements in old-age fertility over many generations,” said Duncan Gillespie, from the Department of Animal and Plant Sciences at the University of Sheffield.

SLIDESHOW: Fertile Over 40

The study looked at the marriage patterns of 1,591 women and used Finnish records from the 18th and 19th centuries. The researchers found that, in the past, women were most likely to be married between the ages of 30 and 35 and tended to reproduce early because they often had no opportunities to do so at an older age.

Today, women don’t start childbearing as early because they often put off marriage until they are older, Gillespie said.

"Now family-building appears to be increasingly postponed to older ages, when relatively few women in our evolutionary past would have had the opportunity to reproduce. As a result, this could lead to future evolutionary improvements in old-age female fertility."

"Childbearing within a relationship is still the norm in modern society, but at ages where fewer women have the chance to reproduce, we should expect the evolution of lower fertility," he added.

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