Women who delay motherhood until their forties are at greatly increased risk of suffering difficult pregnancies, miscarriage or dying during childbirth, a leading obstetrician has said.

Women who delay motherhood until their forties are at greatly increased risk of suffering difficult pregnancies, miscarriage or dying during childbirth, a leading obstetrician has said.

Doctors are failing to tell women about the risks involved in having children later in life, according to Professor Michael de Swiet, an obstetric physician at Queen Charlotte's Hospital in London. He said doctors should "stop playing God" by allowing women in their mid-forties and over to have fertility treatment, and called for more debate on the personal and social implications of older parents.

The number of women having their first child when they are 35 or over has more than tripled in the past 15 years. In 1990, just 3 per cent of first-time mothers were over 35, but by 2002, the proportion had risen to 10 per cent. For women over 42, more than half of all pregnancies end in miscarriage or stillbirth.

Professor De Swiet said that for women aged 20 to 24 in Britain, the risk of dying during childbirth was just 7.3 per 100,000. But for women over 40, the rate rose to 35.5 - a level comparable to countries such as Armenia and Iran.

Women who conceive in their forties are also at increased risk of hypertension - high blood pressure which can lead to pre-eclampsia, a condition which can be fatal for both mother and unborn child.

The risks even affect women who start having children in their early thirties, when there is less risk, but go on getting pregnant as they reach their forties.

A woman over 35 who already has at least two children and then becomes pregnant again is 100 times more likely to die from a blood clot than a first-time 20-year-old mother.

Professor De Swiet, who specialises in treating complications during pregnancy, said: "I have had 90 women in my clinic in the last year over the age of 40 and I do have concerns. There are worries about miscarriage, chromosomal disorders like Down's, high blood pressure and diabetes.

"At the moment, doctors are not telling women about the risks, and even when they do, the women often don't take it in. What you have to remember is that some of these women who become pregnant with IVF techniques are fundamentally unwell - they are not good breeders and they are at high risk of both morbidity and mortality."

It was not simply life-threatening conditions that affected older expectant mothers, Professor De Swiet said. "They seem to be more at risk from what I call the misery factor during pregnancy," he explained. "They tend to suffer more from breathlessness, heart palpitations and fainting. Often by 35 weeks, they have had enough and come in demanding a Caesarean."

He said that women should ideally have their children between 25 and 35, be aware that between 35 and 45 they were "safe enough" but that over that age, they should be made fully aware of the dangers.

He added: "I don't think it is for us as doctors to play God. Society must discuss as to how we should judge who should have pregnancy at an extreme age."

In a separate study, scientists found that women who use donated eggs from non-family members to become pregnant are at increased risk of miscarriage and pre-eclampsia. They suggested that, where possible, women who cannot use their own eggs should ask their sisters to donate in order to reduce the risks to their health and to their unborn child.

Older women in particular often need to use donated eggs because their own are not good enough for IVF procedures.

Researchers in Korea studied 61 women who became pregnant using donated eggs and compared them with expectant mothers who underwent IVF treatment using their own eggs.

The scientists believe eggs from an unrelated donor may lead to the introduction of "foreign elements" to the developing placenta and trigger abnormal responses.

Genetic link to late motherhood

When Cherie Blair gave birth to her fourth child at the age of 45, the Prime Minister was widely admired for his virility and uniqueness at becoming a father while in office.

But it may be that the conception of Leo, now five, was down to Mrs Blair's genes rather than her husband's potency. Scientists have identified a certain type of genetic make-up in women who have continued to be able to become pregnant naturally over the age of 45.

Researchers from the Hadassah University Hospital in Jerusalem studied 250 Ashkenazi Jewish women, all of whom had children in their late forties and all of whom did not use contraception. Eighty per cent of the women in the study had at least six children, as well as a low miscarriage rate.

The researchers tested eight of the women and studied their genetic make up compared to a control group of non-Ashkenazi women. They found the Ashkenazi women had a pattern of gene expression which appeared to protect against DNA damage and cell death in the ovaries.

Dr Neri Laufer, who led the research, said his team had proved that the "pregnancy" genes were not unique to these particular Jewish women because they had also been found in Bedouin women.

He added: "We hope that this will lead to better understanding of the ageing process and improve our ability to preserve fertility in older women."