By Branwen Jeffreys
BBC News Health Correspondent
Obesity is the fastest growing cause of women dying in pregnancy or childbirth in the UK, a report shows.
More than half the 295 women who died during or after pregnancy between 2003 and 2005 were overweight or obese.
Experts say the number of deaths - from a total of two million pregnancies - is low but the trend is very worrying.
The Confidential Enquiries into Maternal Deaths (CEMACH) report calls for more support and advice for obese women before and during pregnancy.
Gwyneth Lewis, CEMACH director and the government's maternity tsar, said the figures showed that childbirth was very safe in the UK.
But she said the growing evidence of a link to obesity was a cause for concern.
The figures suggest that a modest amount of extra weight in pregnancy carries little extra risk, but obesity poses a significant problem.
Fifteen per cent of the mothers who died were morbid or super-morbidly obese.
Dr Lewis said: "Obese pregnant women are probably at four or five times greater risk of suffering maternal death than a woman of normal weight - and the same for their babies dying."
She is concerned many women are not aware of the risk associated with obesity.
Overall, the UK has one of the lowest rates of maternal death in the world.
However, the death rate in the UK has begun to rise. In 2003-05 it stood at almost 13.95 per 100,000 births, up from 13.07 in 2000-02, and just 9.83 in 1985-87.
With obesity levels predicted to soar experts say it is vital that women are fully aware they should try to get to a healthy weight before trying to conceive.
The report says excess weight not only puts a woman at risk of medical complications, it can mask symptoms and cause logistical problems.
In one case, there was a delay in spotting that a woman was at risk of seizures, because a blood pressure cuff could not fit around her arm.
At the Queen Elizabeth Hospital in Gateshead, consultant obstetricians are already seeing a shift in their clinics and delivery suites.
Dr Helene Brandon said around a third of the pregnant mothers they see are obese, and in an average year they care for several women with a body mass index (BMI) of more than 50.
Dr Brandon's maternity unit has a new operating table
That would place those women in the highest risk category of super-morbidly obese.
The maternity unit has had to buy a new operating table that can hold up to 40 stone (254kg) in weight.
It is regularly in use, as around half of obese pregnant women end up having a Caesarean section.
The operation usually requires at least one extra assistant for the surgeon to help manage the bulk of the patient.
Dr Brandon said obese patients face higher risks of certain complications.
She said: "The most common ones are dangerously high blood pressure, heart disease such as angina which could cause a heart attack in pregnancy - and obese patients are much more prone to bleeding."
Struggle with weight
Several miles away on an estate in Jarrow one of her patients, Maria Thornton is working hard to reduce the risks to herself and her baby.
Maria is six months pregnant and weighs 19-and-a-half stone (124kg).
She has had a lifelong struggle with her weight, and had gastric band surgery several years ago.
In her last pregnancy, Maria developed diabetes because of her weight, which brought home to her the risks to her own health from obesity.
Although she is now well informed about the potential health problems Maria is shocked that obesity is emerging as a factor in maternal deaths.
She said: "It is really quite scary. You know there are risks, but you don't think of them in terms of fatality - you kind of think it makes the pregnancy harder or you've got more obstacles to overcome.
"But to hear that it can cost you your life at the end of the day is quite frightening."
Now Maria is going several times a week to the sure start scheme in Perth Green to use the gym.
As a result of regular exercise she has only put on three pounds (1.36kg) in this pregnancy, following the medical advice that obese women should aim to maintain their weight.