Fertility clinics should defer treating obese women until they have lost weight through dieting, exercise or surgery, according to guidelines published today.

Under the recommendations, clinics are advised to begin treatment on severely overweight women only once they have reduced their body mass index (BMI) to below 35. Women under 37 years of age should reduce their weight further, to a BMI of less than 30, the guidelines state.

Obesity dramatically lowers a woman's chances of conceiving and raises the risk of serious complications during pregnancy. A woman with a BMI above 35 is half as likely to become pregnant as a woman whose BMI is less than 30.

Being overweight during pregnancy increases the risk of miscarriage, diabetes and hypertension as well as leading to a greater chance of birth defects in the newborn, including spina bifida and heart defects.

The British Fertility Society, which represents fertility professionals, issued the guidelines to clinics after reviewing academic studies on the issue. Their report is published today in the Journal of Human Fertility.

Adam Balen, a co-author of the report and professor of reproductive medicine at Nuffield Hospital, Leeds, said: "What we're saying is that all women, regardless of their body weight, should be seen and managed by fertility clinics and that those who need to lose weight should be supported in doing so."

Clinics should take an active role in helping patients improve diet and exercise and should advise them on other options, including weight-reducing drugs, such as Orlistat, or gastric bypass surgery, which is only recommended for the severely obese.

Women with a BMI over 30 - deemed obese - do not usually qualify for fertility treatment on the NHS, but 75% of IVF in Britain is paid for in private clinics which have less strict rules.

Bill Ledger, professor of reproductive medicine at Sheffield University, welcomed the guidelines. "If you're slightly overweight it doesn't matter much, but people who have a BMI over 35 are at a really high risk of all kinds of problems if they become pregnant. Doctors shouldn't be helping women have a pregnancy that's at a high risk of going horribly wrong," he said.

One difficulty fertility clinics face is how to treat obese women in their late 30s. After the age of 35, a woman's fertility goes into a steady decline, so they may not have long to attempt to lose weight.

"This isn't about preventing obese women from having fertility treatment, it's about ensuring they have the safest, most effective treatment," Balen added.

In September, a report in the medical journal, the Lancet, warned that soaring obesity levels in the western world are expected to trigger an infertility crisis among women. Some experts believe obesity may be a factor in driving one in five couples to seek fertility treatment within a decade.

The guidelines are not enforceable but fertility specialists expect them to be incorporated next year into the official recommendations of the National Institute for Health and Clinical Excellence.

· This article was amended on Thursday November 15 2007. The British Fertility Society's report about obesity and fertility is published in the Journal of Human Fertility, rather than the Journal of Human Reproduction as we said in the article above. This has been corrected.