Obese women should be refused fertility treatment until they lose weight, IVF say experts.
The argument is that obese women are more likely to suffer complications
Whether they are seeking treatment on the NHS or privately, professionals are being urged to deny treatment to women with a Body Mass Index of more than 35.
Where possible, the British Fertility Society says IVF should be offered only when her BMI has dropped below 30 - a figure based on both height and weight.
One obesity expert branded the new guidelines "discriminatory".
But the British Fertility Society said its new advice was based on what was best for both the mother and her baby.
"Obesity reduces the chances that a woman will conceive naturally and decreases the possibility that fertility treatment will be successful," said Mr Tony Rutherford, the chair of the BFS's policy committee.
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
"It also increases the risk of complications during fertility treatment and pregnancy and endangers the health and welfare of both mother and child."
Among the "complications" listed was the difficulty of providing safe anaesthesia for obese women during procedures, as well as problems with viewing ovaries on an ultrasound scan.
Obesity is also thought to raise a woman's risk of miscarriage after IVF treatment.
The BFS guidelines have no legal weight, but they are referred to by many professionals working in the field.
The chief executive of the Infertility Network endorsed the recommendations, but stressed women must be offered the help they needed to lose weight.
"We would also recommend that clinicians adopt a flexible approach where possible and look at cases on an individual basis," said Clare Brown.
But the chairman of the National Obesity Forum, Dr Colin Waine, said he found the new guidelines "troubling".
"Weight loss may improve the success of treatment, and women should be made aware of that, but to deny treatment outright is discriminatory," he said.
"We are getting ourselves onto an increasingly slippery slope of rationing on the basis of weight, even when overweight people may benefit immensely from treatment."
Some 25% of UK women are said to be obese, or with a BMI of 30 or more.