Scientists track changes in fertility as women age

Academics Duncan Gillespie, Dr Virpi Lummaa and Dr Andrew Russell, from the university's department of animal and plant sciences, studied Finnish church records from the 18th and 19th centuries.

Using those records, which date from a time when almost everyone married and divorce was forbidden, the researchers traced the survival and marriage histories of 1,591 women.

They found that women aged between 30 and 35 were the most likely to be married and women who married wealthy husbands tended to marry at a younger age but to older men so they were more likely to be widowed.

Coupled with low remarriage prospects for older widows with children, this high chance of widowhood reduced the number of women who had the opportunity to reproduce at an older age.

Nowadays, however, women are older when they have their first children and short-term relationships and divorce are more common.

Consequently, the researchers found, natural selection maintaining fertility at a young age may weaken, and natural selection favouring reproduction later in life could increase. Over many generations, this could lead women being more fertile in their 40s.

Mr Gillespie said: "In today's society, 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."