Half the women of reproductive age in the UK are damaging their chances of having a baby by allowing themselves to get too fat, it was claimed today.
The scale of the problem was highlighted by an IVF study which found that being overweight doubled the risk of women miscarrying.
This was despite each patient having a healthy, potentially viable embryo transferred to her womb after successful fertilisation.
Miscarriages are already known to be more common among overweight women who conceive naturally.
The new findings, focusing on healthy IVF embryos, suggest that weight and not some other factor is probably responsible for the failed pregnancies.
Around half of reproductive age women in the UK are overweight, as defined by their Body Mass Index (BMI), which relates weight and height.
More than a third may have BMI scores of 30 or above, placing them in the obese bracket.
Commenting on the research, leading British expert Professor Adam Balen, from the Leeds Centre for Reproductive Medicine, said: "About 30-35% of women of reproductive age in the UK are obese.
"A large proportion of women are jeopardising their chances of having children. It's an extremely important issue."
Results from the study, conducted at Guy's and St Thomas' Hospital in London, were presented today at the annual meeting of the European Society of Human Reproduction and Embryology (Eshre) in Rome.
Researchers led by Dr Vivian Rittenberg looked at 318 women undergoing IVF who were divided into two groups at the start of treatment according to their BMI.
One group of 185 had BMIs in the normal range from 18.5 to 24.9. The other was made up of overweight and obese women with BMIs of 25 or above. Of these, 14% were obese.
Overall just over a quarter of the women miscarried before the end of their 20th week of pregnancy.
After adjusting for other factors such as age, duration of infertility, smoking, and miscarriage history, the researchers were able to show that being overweight or obese more than doubled the risk of miscarriage.
Dr Rittenberg said: "We found that women who had a BMI of over 25 had a one in three chance of miscarriage compared with women with a normal BMI whose chance was one in five."
She pointed out that previous studies had produced conflicting evidence of the effect of excess weight on IVF pregnancy outcomes.
"Our study differs in that we transferred only one embryo at a specific age of development, and were therefore able to provide clear evidence of the deleterious effect of being overweight on the chances of miscarriage," Dr Rittenberg added.
Dr Tarek El-Toukhy, another member of the Guy's and St Thomas' team, said: "Overweight women wishing to get pregnant by spontaneous conception are already counselled to lose weight before trying for a baby.
"Our findings have shown clearly that women undertaking ART (assisted reproductive technology) should be strongly encouraged to heed this advice in order that they can have the best possible chance of obtaining and maintaining a pregnancy."
Scientists are still trying to find out why being overweight can lead to miscarriage.
One theory is that overweight women have higher levels of insulin in their blood, which may alter the lining of the womb and prevent implantation of the embryo.
Being overweight is also associated with other pregnancy problems, including high blood pressure, pre-eclampsia, diabetes, premature delivery and bleeding.
Babies of obese mothers are more likely to be large, necessitating a Caesarean delivery which carries risks in itself.
The professional guidance to fertility clinics in the UK says they should avoid offering IVF treatment to women with BMIs of over 35.
Primary care trusts which provide NHS funding for IVF will not normally pay for the treatment of obese women.