The modern career woman who delays starting a family into her thirties is defying nature and risking heartbreak, doctors say today. Women who try to have it all may lose the one thing that matters most, the opportunity to have a family, they say.

In the starkest warning yet about the dangers of older motherhood, the authors, three senior obstetricians from Guy's and St Thomas' Hospital in London, catalogue the problems that can befall older women who try to become pregnant, ranging from high blood pressure to diabetes and foetal abnormality.

Men are also affected by delay because sperm counts decline with age and the risks of schizophrenia and genetic disorders in their offspring increase.

Many couples wrongly believe they can fall back on IVF if all else fails. But this "expensive, invasive" treatment has a high failure rate, the doctors say. Seven out of 10 women who have IVF do not achieve a live birth, rising to nine out of 10 over the age of 40.

The obstetricians say in the British Medical Journal: "Women want to have it all but biology is unchanged ... If women want room for manoeuvre they are unwise to wait till their thirties."

The age of first motherhood has been rising since the mid-1970s, with the sharpest increases in births to women in their late thirties and early forties, but the "biologically optimal" time for childbearing is 20 to 35. Although the risks to individual mothers are still low, the rate of complications and abnormalities is increasing. Doctors including Lord Winston, the fertility pioneer, have warned about the trend but they focused on older mothers.

Susan Bewley, consultant obstetrician at Guy's and lead author of the BMJ editorial, 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 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 half say 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."

A quarter of patients at Guy's were over 35 and the proportion was higher in other hospitals. Social and psychological benefits of older motherhood were slight by comparison with the physical risks. Children of older parents tended to do better at school but this could be because older parents tended to be better off and better educated.

Dr Bewley added: "There is a sense that we are healthier than we have ever been and it's OK to wait, but as obstetricians we see people falling off the cliff and it saddens me."

The warning was backed by Infertility Network UK, a charity that supports couples with fertility problems. Clare Brown, the chief executive, said: "Delaying having children until you are in your thirties is a choice many women make but they need to be aware of the problems."

Marion Frostick, 47, a management trainer in Oxford, spent a decade trying to get pregnant from the age of 30. "I had been working and I was enjoying my career and I thought it could wait. But when I tried, it didn't work."

She started on three years of tests and investigations, the stress of which contributed to the breakdown of her first marriage. After remarrying at 36, she was still unable to get pregnant. Her last resort was IVF.

"I was one of the lucky ones: I got pregnant first time. I thought I would have another go at 41 and again I was incredibly lucky." But Ms Frostick is sceptical about telling women they should start their families by 30. "It may the right advice medically but things happen. Real life is not like that."

The Infertility Network can be contacted via its website,

Baby in your 40s? 'Go for it. It's good fun. Honest'

Emma Burstall, journalist, 44

"You're what?" friends said when I told them that I was expecting a third baby. "You must be mad."

At approaching 40, and with two children of nine and 15, everyone assumed I'd completed my family. But what most didn't know was that for some time I'd had this niggling feeling that I hadn't done with babies.

For a variety of reasons, I'd put off making any decisions but now, as my 40th birthday loomed, I realised if I didn't get a move on, it would be too late. My husband took persuading but fortunately, came to share my vision.

I was lucky. I had no trouble getting pregnant. But I can't imagine how I'd have felt if it hadn't happened. I would have been desperately disappointed, and I'd have kicked myself for not getting my act together sooner. I knew it was a possibility I wouldn't conceive. Two friends had first babies in their mid-thirties and badly wanted a second. But they'd left it too late.

There's no doubt I was more concerned about Down syndrome but I chose to have an amniocentesis, which confirmed everything was fine, and Freddie arrived safely in January 2002. He seemed to slot into family life with remarkable ease.

So, as it turned out, I had my daughter, Georgia, in my twenties, Harry in my thirties and Freddie in my forties. Freddie's birth was no harder, my recovery no slower, and I haven't felt more tired being an older mother; in fact, I would recommend it.

It's been good to feel, dare I say it, slightly wiser, certainly more experienced, and better off financially than I was when I had my first child. Also, being more established in my career, I was able to negotiate three days a week at Family Circle, where I'm features editor.

To anyone considering having a late baby I'd say, "Go for it. It's good fun. Honest". But if you'd be devastated if it didn't happen, don't hang about. Because that "autumn crocus", as someone once described their post-40 child to me, might not come along.