According to new research, by the age of 30, a woman's chances of conceiving have dwindled dramatically. But it's not just about statistics, says Julia Llewellyn-Smith.

Feeding her four-week-old baby Oliver, Helen Scurrah is the picture of a contented new mother. "When I met my husband, I knew I wanted to have children, but we wanted some time for ourselves first," she says. "Then after three years, I began to think, 'I really would like to have a baby. This is very important.'

"We started trying but then stopped because we were getting married, and I didn't want to be pregnant on my wedding day. Then on the honeymoon I got pregnant straight away. I feel very lucky."

Scurrah, from Poole, Dorset, isn't just lucky to have a healthy son. At 41, she had postponed trying to conceive until an age when her chances were worryingly slim.

"Everywhere I looked there were headlines saying it wasn't going to happen, but it did. I had a straightforward pregnancy, even though I had a label in hospital saying 'Low risk (but 40)', which made me chuckle."

Fertility experts would consider Scurrah's attitude to motherhood shockingly laid-back. Last month, the first study was published tracking the supply of eggs from the ovaries from a woman's birth to the menopause. Researchers at St Andrews and Edinburgh universities discovered that, by the age of 30, an average woman has just 12 per cent – barely an eighth – of the eggs with which she was born. By 40, only three per cent of the two million or so original eggs remained.

The message was clear. If you want children, it's best to start as early as possible. Yet, as Scurrah, a member of the online support group mothers35plus, points out, life is rarely that simple.

"These warnings presume an awful lot – that a woman has a fantastic career and has made a conscious choice not to have babies because of her lifestyle. But usually a woman is childless because she hasn't met the right man, because she's not in a financial position to have a baby, or she's just not ready."

A couple of generations ago, it was far more straightforward. You left school, married soon afterwards, and babies almost inevitably followed. Today, however, many twentysomething men and women are saddled with student debt and are still living with their parents.

"The middle classes do not aspire to having children young," says Sally Gimson, of the Family and Parenting Institute. "Both men and women want to go to university, pursue a career and buy a house before they settle down, which is almost impossible to do before you are 30. Teenage pregnancies are strongly discouraged."

The average age for a British woman to have her first baby is now 29. At Queen Charlotte's maternity hospital in west London, with an affluent, urban catchment, the average age is 35. The waiting room is full of women frowning over their BlackBerrys, cancelling their Botox appointments.

Helen Fielding, the creator of Bridget Jones, the prototypical career-girl singleton, had her children when she was 46 and 48, while dozens of celebrities, from Madonna to Cherie Blair, have become mothers in their forties. I became pregnant at 35 and 37, with no problems.

Surrounded by such examples, many assume that female fertility has adapted to society's drastic changes. Not so, according to Dr Tom Kelsey, the researcher of the St Andrews study. "A woman's ovarian reserves are the same now as they were in the Stone Age. I'd be horrified if one of my teenage daughter's friends became pregnant, but biologically that is exactly what she should be doing.

"There are so many women putting off babies for social and cultural reasons who would have big problems conceiving even if they started now. I've worked in an assisted-conception unit full of women in their thirties and forties, who were desperate to get pregnant but couldn't. If they'd started earlier, they could have had a large family. Yet the fastest rising number of women having abortions are in their thirties. It's a huge risk given that it may be their final chance to conceive, but they seem to think they have forever."

The birth rate is still rising in the UK, but one in five women born in 1963 is childless, compared with one in eight born around 1933. The number of couples having IVF is rising by about five per cent a year. One in six couples will have difficulty conceiving, of whom one in four will have successful fertility treatment.

"That means three in four don't succeed," says Susan Seenan of the Infertility Network. "Of them, some will keep trying and some will adopt. But many will give up and be involuntarily childless. The financial price of the treatment may be too high, or it may be that the emotional and physical cost is too much to bear."

Dr Kamal Ahuja, scientific and managing director of the London Women's Group, the country's largest fertility clinic, reassures clients that, for women younger than 37, his clinic has success rates of about 50 per cent.

After that, however, the news is bleak. Between the ages of 35 and 40, about 30 per cent succeed; after 40, this declines to about 12 per cent. "After the age of 42, the chances of having a baby with your own eggs are minuscule. You will need egg donation."

Researching my latest novel Love Nest, which has a theme of infertility, dozens of women in their late thirties and forties told me about the hell they had been through to conceive. Some had spent tens of thousands of pounds on several rounds of IVF; those who succeeded had often had twins. Pregnancies were fraught with miscarriage or fetal abnormalities, such as Down's syndrome.

One, aged 40, tried IVF in her quest for a second baby but was told her eggs were of too poor quality for it ever to succeed. She is now resigned to her four-year-old daughter growing up an only child.

Several had gone to clinics abroad for egg donation. Their partners' sperm was combined with strangers' eggs and implanted in their wombs. "It worked on the second attempt, but I have to accept that we'll only have one child, which breaks my heart, and that I'll probably be too old to be any use as a grandmother," says Christine, who had a son at 45.

The consequences are not just for individuals but for society. Late parenthood means a surplus of old people and not enough workers, which could further stymie the economy. On the other hand, more teenage mothers would mean a population – and economic – boom. "Women will presumably finish having their families when they are younger and then have more years of employment ahead of them, their children will also come into the labour force and add to growth," explains economist Dr Ros Altmann.

As she winds Oliver, Scurrah laughs when I ask if she could imagine having had him at 25. "No way. I can't imagine it at 35. There are advantages to having children young, but I was having too much fun. I'm glad I waited, but that's easy to say with hindsight. I certainly wouldn't have wanted to end up childless."

* 'Love Nest' by Julia Llewellyn (Penguin Books) is available from Telegraph Books (...).

Source: Telegraph.co.uk - http://tinyurl.com/yanapu6