London, Jul 23 (IANS): The trend for having children later in life could lead to women becoming more fertile in their 40s, researchers at a British university have suggested.

As many women wait until they are older to give birth, only those with longer lasting fertility genes will be successful. That means the average length of fertility will grow as their genes will be passed on to their children.

The study from the University of Sheffield shows that previously women would marry early and if widowed were too old to remarry thus favouring early childbirth in women, The Daily Telegraph reported Thursday.

But today's reluctance to settle down and reproduce until later in life could lead to fertility beginning to favour older women.

The scientists studied marriage patterns to trace the survival and marriage histories of 1,591 women.

Using 18th and 19th century Finnish records - a time when almost everyone got married and divorce was strictly forbidden, they found that women aged 30 to 35 were the most likely to be married.

Those that wed wealthy husbands were married at a younger age but to relatively older men, meaning family sizes were bigger but with an increased risk of widowhood.

The researchers say that this high chance of widowhood, coupled with low remarriage prospects for older widows with children, limited the percentage of women in the population with the opportunity to reproduce at older ages.

Duncan Gillespie, from the university's department of animal and plant sciences said that in today's society however, women do not start childbearing until an older age as marriage is often delayed.

"As a result of this, 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," he 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.