Celebrities give the impression that getting pregnant in your forties is child's play, but for the rest of us it's not always so easy.

Nicole Kidman is pregnant. After suffering a miscarriage and an ectopic pregnancy while married to Tom Cruise, the 40-year-old Hollywood star declared last week that she is expecting again, with her current husband, the country singer Keith Urban.

The couple were pictured in Sydney after Kidman had reportedly dropped plans to shoot a film in Germany in order to protect her unborn child. She joins a growing list of celebrities who have sneaked motherhood in under the wire. Carla Bruni, the French president Nicolas Sarkozy's fiancée, is rumoured to be pregnant with her second child at 40. Madonna gave birth at 41; Holly Hunter had twins at 47; and Cherie Blair produced Leo, her fourth child, at 46.

Are these women exceptional? How easy is it to get pregnant in your forties? How many resort to IVF with donor eggs? Doctors have warned that women who delay motherhood are "defying nature" and increasing the risks for themselves and their babies, provoking a backlash in some quarters from women who smell a conspiracy against older mothers.

Daisy Waugh, the TV presenter and a first-time mother at 39, attacked the double standard whereby ageing rockers Paul McCartney and Rod Stewart, who both fathered babies in their sixties, are congratulated with a slap on the back and a nod and a wink, while "old girls" like her are "gently encouraged to worry".

"You keep at it, old boys! Breed away! I just wish people weren't so antsy about the old girls, now that we're doing the same thing... We are fed a constant drip of negative, alarmist stories about the dangers of delaying motherhood, and I can't help it, I smell a rat."

What has really irritated Waugh and women like her is pronouncements by gynaecologists such as Dr Susan Bewley, a consultant at St Thomas' Hospital in London, who declared that women who put off childbearing until their late thirties or beyond were risking disaster. Writing in the British Medical Journal, Dr Bewley and colleagues said: "If you want a family – and most people want a couple of children – and you are going to complete your childbearing by 35 and leave time for recovery in between, you would be wise to start before 30 ... People are aware that ageing is a bad thing but the bio-panic women had on their 30th birthday has moved up to the 40th birthday. Surveys of older mothers show that half say that they delayed because they had not met a suitable partner. Maybe instead of waiting for Mr Right they ought to wait for Mr Good-Enough, if they want children."

The article caused a storm when published in 2005, and it's still reverberating today. Dr Bewley was recently forced to apologise on the website Mothers35plus.com after she compared the problem of the post-40 mother to the pre-20, teenage one. "The last thing I want to do is insult anyone," she wrote. "However, my colleagues and I have been concerned about the increasing distress and complications we are seeing in our professional lives. There is a rising amount of infertility, miscarriage and complications of pregnancy as the average age of childbearing goes up."

The comparison to teenage pregnancy was to point out that there were problems at each end of the reproductive spectrum, one more social, the other more medical. While the number of teenage pregnancies is falling, those in women of over 35 were much higher and rising dramatically. "What I am concerned about is why, over the last 30 years or so, women are having babies later and later, with more risks, when we are generally much more risk-averse nowadays."

Dr Bewley is not alone. Many obstetricians who daily witness the heartbreak caused by infertility warn of the risks to women who "want to have it all, when biology hasn't changed". Sam Abdalla, director of the Lister Fertility Clinic, says that society is imposing a "massive strain" on women by forcing them to choose between family and career. "It puts more of a burden on the women because it reduces their chances of conceiving and puts more strain on [IVF] treatment. In women over 40, treatment is less successful, with fewer pregnancies, a high miscarriage rate and a lower live birth rate."

It comes down to statistics. Fertility peaks in the mid-20s. At 30, three-quarters of women will get pregnant within a year, but this falls to two-thirds at 35. Although most pregnancies over 35 go very well, a minority of women run into problems, some very serious.

By 40, less than half (44 per cent) will get pregnant and have a baby within a year. By this age, a woman who doesn't conceive quickly – assuming she has regular intercourse – will be anxious about time running out, and thinking urgently about IVF, if she can afford it. Even conception is no guarantee of a live birth – half of all pregnancies over 40 end in miscarriage.

Many childless women watching the years pass reassure themselves that there is always IVF to fall back on. But this, too, often turns out to be a mirage. The fortysomethings are the fastest- growing group seeking IVF, up from less than 1,000 women in 1991 to more than 6,000 in 2006. But for the vast majority, treatment will end in failure as doctors run up against the barrier of the biological clock. Over the age of 43, more than 95 per cent of those trying IVF go home empty-handed. Even among those who are successful, half use donated eggs, so aren't the genetic parent.

Several celebrities who have had babies in their forties are thought to have had IVF using donated eggs, but few admit to it, fostering the impression that it is still perfectly easy to become a mother of your own genetic child into your fifth decade. Whenever an older celebrity has twins – Geena Davis did so at 48, Holly Hunter at 47, and Jane Seymour at 45 – the gossip turns to egg donation.

Among younger women, medical advances and improvements in technique have seen success rates for IVF treatment improve dramatically, and overall live birth rates have risen from 14 per cent per cycle in 1991, to 21 per cent in 2006. But success rates fall rapidly past the age of 35, declining to 12 per cent per cycle at 40. Disappointingly, the over-40s have not seen the improvement in live birth rates experienced by younger women. While modern IVF techniques can rejuvenate an older womb and prepare it for pregnancy, they have not yet succeeded in pulling off the same trick with older eggs.

This is the untold story of late motherhood. Among young women, IVF has matched and, in some clinics, bettered birth rates achieved by natural conception, but in older women it has only done so using younger women's eggs, of which there is a shortage, with growing demand and lengthening waiting lists. Some women have frozen their eggs for later use, usually while undergoing cancer treatment, but the technique is still in its infancy and only a few hundred babies have been born worldwide from frozen eggs. With a few notable exceptions, IVF has failed significantly to extend most women's reproductive lives.

There are other problems an older mother faces. All the complications of pregnancy, from birth defects to premature delivery and Caesarean birth increase with age. The risks, however, may not be confined to women. Older fathers, in their 40s or above, are six times more likely to father a child with autism, according to a 2006 study published in Archives of General Psychiatry. The risk of autism rose steadily with advancing paternal age, but there was no link with increasing maternal age, suggesting that men may have their own biological clock.

Ultimately, women (and their partners) have to make a choice. Do they opt, with Daisy Waugh, to "ignore the doom-mongers, laugh at the statistics, lie back and think of Madonna". Or do they, with India Knight, the Sunday Times columnist, complain that women "have been fed a complete lie": that they can have it all, and children, too, when "the truth is there is a hurry and, like it or not, biology does discriminate".

It's not an easy choice, and there are no easy answers. But if you really want a baby, there may be no time like the present.