Soaring levels of obesity in the western world are expected to trigger a major new infertility crisis among women, doctors warn today. The Lancet reports that the obesity epidemic will leave more couples struggling to conceive as women suffer more fertility-related problems.

One expert suggested the proportion of couples seeking infertility treatment could double to one in five within a decade. "We're at the tip of the iceberg," said Bill Ledger, professor of obstetrics at the University of Sheffield. But he said the problem could be eased if women lost weight before resorting to medical interventions to get pregnant.

The Lancet study focuses on a common disorder - polycystic ovary syndrome (PCOS) - which affects one in 15 women worldwide. PCOS occurs when women have excess male hormones which cause little cysts to grow on the ovaries. Symptoms include irregular or missed periods, infertility, excess hair, acne and insulin resistance which can make them more prone to diabetes.

"Obesity has a substantial effect on the manifestation of polycystic ovary syndrome," the paper says. "Excess weight exacerbates metabolic and reproductive abnormalities in women with the syndrome, and family studies suggest weight gain might promote ... polycystic ovary syndrome in a susceptible population."

Robert Norman, the lead author at the University of Adelaide, Australia, said: "Obesity doesn't cause PCOS but it exacerbates it ... it's argued that obesity will cause a crisis in infertility and I agree.

"Obesity has huge consequences for fertility but also when you treat someone who is obese and get them pregnant you have increased problems for the pregnancy." Problems include pregnancy-related diabetes, abnormalities and a decreased chance of a natural labour.

Adult obesity rates have nearly quadrupled in the UK over the past 25 years, and two-thirds of adults are overweight; of these, nearly a quarter are obese.

Prof Ledger said obesity was a big factor in predictions that the number of couples seeking fertility treatments would double in the next 10 years. "Given the rise in obesity incidence it's common sense to expect a rise in subfertility. Prescriptions for drugs which help women with PCOS get pregnant are going up."

Around 7% of women in the UK have PCOS. This is expected to rise to 10% in the next decade. "It's a common problem, but the biggest thing is to tackle obesity. It's a problem that needs to be dealt with in lifestyle changes rather than through surgery. Losing weight will help women with PCOS get pregnant."

Women with a body mass index of over 30 - deemed obese - are usually not allowed NHS infertility treatment until they have lost weight. But most IVF cycles in the UK are paid for in private clinics which have less stringent rules.

Today's paper also reveals that if women lose even a little weight and, more importantly, improve their fitness, their chances of getting pregnant radically improve.

Geeta Nargund, a fertility expert and leading proponent of milder, safer forms of IVF, said: "I would be concerned if any clinic was offering IVF to overweight women without first advising them to lose weight. Fertility treatments aren't just about getting women pregnant but about healthy mothers and healthy children."

Clare Brown, chief executive of Infertility Network UK, which supports people with fertility problems, said: "If a patient has made every effort to lose weight it would be nice to have a flexible approach."