Scientists have warned, women who decide to delay their first baby until their late-30s are running the risk of being left childless because of age-related infertility.

Scientists have warned, women who decide to delay their first baby until their late-30s are running the risk of being left childless because of age-related infertility. Matthew Ennis Photography

YOUNG women who decide to delay their first baby until their late-30s are running the risk of being left childless because of age-related infertility, scientists have warned.

A woman's fertility declines dramatically after the age of 35 and an increasing number of women are finding that they have to turn to IVF treatment to have any hope of becoming pregnant, experts said.

"Things are really going downhill between the ages of 30 and 40....the best cure of all is for women to have their babies before this clock strikes twelve," said Professor Mary Herbert, a reproductive biologist at Newcastle University.

"They should think of family planning not just in the context of preventing pregnancy but also think of it in the context of having your babies at a time when you still have your reproductive fitness...I would be worried if my own daughter didn't have a child at 35," Professor Herbert said.

The number of British women between the ages of 35 and 39 giving birth for the first time has increased from 6.8 per cent in 1986 to 17 per cent in 2008. This has been accompanied by significant increase in the number of women over 40 seeking fertility treatment, many of whom will ultimately fail to become pregnant, the experts said.

Fertility specialists are increasingly concerned about the trend towards having children later in life because, although it can work for some women, it causes problems for many more because female fertility declines so rapidly after 35.

"Family planning has got to include when you are going to have your children as well and what we are seeing over the years is an increase in the age of women seeking fertility treatment," said Jane Stewart, a consultant in reproductive medicine at Newcastle University.

"We cannot get away from the age-related decline in fertility and I think to put forward the idea of delaying having children is fine for some people but it's not fine for everybody," she said.

"I don't think women understand the risks they run when they reach the older ages....It's better to think about it [in your 20s] and having that thought process inform all possibilities," she told the British Science Festival in Newcastle.

Women are born with a certain number of egg cells in their ovaries and these are gradually lost as they get older, Dr Stewart said.

"Women at the age of menopause have effectively run out of eggs. We do run out of time," she told the meeting.