Women’s health centre

Women have an eighth of their ‘eggs’ left by age 30

28th January 2010 - Scientists have discovered why women who leave it until later in life before trying for a baby may find it harder to get pregnant: their potential egg supply is edging towards empty by the time they reach 30.

A study has tracked the sharp decline in female fertility from the moment of conception to the menopause. By the time a woman has blown out the candles on her 30th birthday she will have lost about 88% of her supply of ‘eggs’.

The research was carried out by Dr Hamish Wallace, a paediatric oncologist from the University of Edinburgh and his colleague Dr Tom Kelsey, a Senior Research Fellow at the School of Computer Science at the University of St Andrews.

Ovarian reserve

They set out to examine the rise and fall of the ‘ovarian reserve’ - the potential number of eggs in a woman’s body. They looked at data from 325 women of various ages and charted their reserves of non-growing follicles (NGFs) which form the pool from which the eggs can be drawn.

Their investigation - which is published in the journal PloS ONE - shows that a woman’s egg pool is largest when she herself is a developing foetus in the womb. At around 18 to 22 weeks after conception, a female has an average population of 300,000 potential eggs in each ovary.

However, Dr Tom Kelsey, says from then on this pool goes into decline. “At 20 weeks post conception there’s a peak in this population,” he tells us. “But then the population of these cells decline, even though the female is pre-birth and not of reproductive age; and they are being lost with no hope of them turning into eggs.”

Birth to menopause

The study shows that the rate at which NGFs are recruited into eggs in a female’s body increases between birth and the age of 14 and then reduces with age until the menopause. The authors say that the start of puberty probably accounts for this change. “Our data suggests that the onset of oestrogenisation and ovulation heralds a slowing in the rate of NGF recruitment,” they write.

The decline of a woman’s egg supply - and her fertility - has been the subject of a number of studies, but this latest investigation is the first to chart the entire process from conception to the end of child-bearing years. As for a woman’s egg supply, “Nearly all women have got an eighth left at age 30 and 3% at age 40,” says Dr Kelsey.

The authors believe that “An increased understanding of the dynamics of human ovarian reserve will provide a more scientific basis for fertility counselling.”

As for women who may be delaying starting a family until later in life, Dr Kelsey says “The message is that while culture changes, human biology doesn’t change at the same rate.”