Quality of eggs also suffer with age, new study reports

LONDON - Scientists have discovered the reason why women find it difficult to conceive later in life - they have used up about 90 per cent of their "ovarian reserve" by the age of 30.

While they may continue to produce eggs throughout their 30s and 40s, the reservoir of potential eggs from which they are taken has shrunk to almost nothing, it suggests.

As the body chooses the best eggs from the reserve, the likelihood is that the quality of the eggs will suffer as women get older, increasing the difficulty of conception and the risk of an unhealthy baby.

The new research by the University of St Andrews and Edinburgh University is the first to collate the actual decline of the "ovarian reserve" - the potential number of eggs women are born with - from conception to the menopause.

It shows that, on average, women are born with 300,000 potential egg cells but this pool declines at a much faster rate than first thought. By the age of 30, there is only 12 per cent left on average and by the age of 40 just 3 per cent.

The researchers said many women make the mistake of thinking that because they are still producing eggs, their fertility remains constant. But this new research shows that it declines rapidly.

Dr Hamish Wallace (Edinburgh), the co-author, said: "Our research shows that they are generally over-estimating their fertility prospects.

"Our model shows that for 95 per cent of women, by the age of 30 years, only 12 per cent of their maximum ovarian reserve is present, and by the age of 40 years only 3 per cent remains."

The researchers also discovered for the first time that the number of eggs in the ovarian reserve peaks at about 20 weeks after conception - when the female embryo is still in her mother's womb - and dwindles until menopause at the age of about 50.

They also found that the rate of recruitment of immature eggs towards mature eggs increases from birth until approximately 14 years of age, then drops off.

Co-author Tom Kelsey, of St Andrews University, was quoted by Britain's The Daily Express newspaper as saying that was something they hadn't seen before.

"We think there is some sort of hormonal explanation that during puberty, something switches and women start losing fertility at a different, faster rate," he said.

The study collected information from 325 different women of different ages in the United Kingdom, the United States and Europe.

The research, published in the journal Public Library of Science One, also showed that there was an enormous difference between the size of individual women's "ovarian reserve". Some women had more than 2 million, while others had as little as 35,000.

The research is the latest to warn that women should not leave it too late to conceive. Women's fertility declines substantially after the mid-30s but the speed of the drop differs for each individual.