An overweight woman's chance of getting pregnant steadily falls as her weight increases, a major study has found.
The study is the first to follow a large group of women
Among 3,000 women with fertility problems, there was a 4% drop in the chance of pregnancy for every body mass unit (BMI) rise above a certain point.
Dutch scientists, writing for the journal Human Reproduction, said that very obese women fared the worst.
The British Fertility Society says some women weight should be barred from IVF on account of their weight.
The level of obesity among would-be mothers is increasing in the UK faster than in almost any other country in the world.
Doctors know that this affects a woman's chances of getting pregnant naturally, but are more worried by the increased risks to her health - and the health of her baby - that the extra weight poses.
The study by researchers at the Academic Medical Center in Amsterdam is the first to follow a large group of women trying for a baby, and to see directly what effect their body mass had on the outcome.
The standard unit of weight is body mass index (BMI), which is the weight in kilograms divided by the height squared.
Anything above 25 is considered overweight, while exceeding 30 is defined as "obese".
All the women in the study had come to see fertility doctors, but there was no obvious reason for their failure to conceive, as they were still ovulating normally.
Some women with "unexplained infertility" do go on to become pregnant naturally.
The study found that there was a clear relationship between their BMI and their likelihood of achieving this.
Compared with women with a BMI between 21 and 29, for every BMI point between 30 and 35 there was a 4% drop in conception rates.
Severely obese women, with a BMI of over 35, were between 26% and 49% less likely to conceive compared with a BMI between 21 and 29.
Dr Jan Willem van der Steeg, who led the study, said: "Given the increased prevalence of obesity, this is a worrying finding.
"We think that women should be informed about their lower pregnancy chances due to their overweight.
"We hypothesise that losing weight will increase the chance to conceive without treatment."
This position is shared by the British Fertility Society, which issued guidelines to its members last month urging them to withhold fertility treatment from obese women until they lost weight.
Mr Tony Rutherford, who helped draw up the guidance, said: "Over the last few years there has been evidence that obesity can harm the fertility of women.
"It decreases the chances of getting pregnant, and increases the risks of pregnancy - to both mother and child.
"Sometimes there are difficult balances to be made, and obviously we do feel for couples who are trying to have a baby."