Average success rates per cycle of IVF are 23%
Overweight and obese women have as much chance of having a baby through fertility treatment as normal weight women, a Scottish study suggests.
IVF treatment is no more expensive for most obese women, the report in the journal Human Reproduction added.
But women should be advised to lose weight because of the high risk of complications, the researchers said.
Most primary care trusts limit IVF to women with a body mass index under 30, which excludes women classed as obese.
The research, on 1,700 women who underwent their first cycle of IVF between 1997 and 2006 in Aberdeen, found 28% were overweight, 8% were obese and 5% had a BMI over 35 - classed as heavily obese.
No significant difference was found between groups in the proportion of women having a positive pregnancy test, ongoing pregnancy, and live birth.
And there was no difference in the cost of a live birth between normal weight women and women with a BMI up to 35.
But a higher proportion of women in the overweight or obese groups had a miscarriage.
And they needed higher doses of drugs used to stimulate the ovaries.
Age 'more important'
Study leader Dr Abha Maheshwari, clinical lecturer in reproductive medicine at the University of Aberdeen, said they had expected costs to be higher in overweight and obese women.
But the study showed patients should not be discriminated against because of their size, she said.
"It shows that age is a more important factor than weight.
"Everybody should be encouraged to lose weight, but treatment shouldn't be declined on weight alone."
She said women with a BMI over 35 should not be offered IVF until they had lost weight because of the particularly high risk of complications.
Underweight: Less than 18.5
Normal: 18.5 to 24.9
Overweight: 25 to 29.9
Obese: 30 or more
BMI is calculated by dividing weight in kilograms by height in metres squared
The British Fertility Society agrees that no one with a BMI over 35 should get IVF, but says that for those with a BMI over 30, fertility treatment should be delayed until they have lost weight unless their age is against them.
Professor Adam Balen, an expert in reproductive medicine at Leeds Teaching Hospitals and author of the British Fertility Society guidelines, said these were put together on clinical grounds, not cost grounds.
"If you look at all the data, there is no doubt that obesity has a powerful effect on fertility but you can overcome it with fertility drugs.
"However, you still have a high risk of miscarriage and it is associated with maternal and foetal deaths."
Dr Virginia Beckett, a spokeswoman for the Royal College of Obstetricians and Gynaecologists, said the study would get people talking but she would want to see bigger trials done before practice was changed.
"There is well-established data that over a BMI of 30 you have a much higher risk of complications."