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Post-menopausal women who are overweight or obese have a much higher risk of developing ovarian cancer, according to a new study.

The research, published in the medical journal Cancer, shows that cancer risk jumped 79 per cent for women who were obese (meaning their body mass index, an approximation of body fat, is 30 or more). Cancer risk increased by 31 per cent for those who were overweight (a BMI between 25 and 30).

The risk was even higher for women who were consistently heavy as adults, compared with those who put on significant weight after menopause.

Michael Leitzmann, a researcher at the National Cancer Institute in Bethesda, Md., and lead researcher on the study, explained that excess body weight causes production of estrogen, which can fuel the growth of cancer cells. Earlier research has shown that women who take birth-control pills have a sharply lower risk of developing ovarian cancer. Oral contraceptives suppress ovulation and the production of estrogen.

And studies have also shown that people who are obese have a higher risk of colon, breast and endometrial cancer. The new study adds ovarian cancer to that list.

"The observed relations between obesity and ovarian cancer risk have relevance for public health programs aimed at reducing obesity in the population," Dr. Leitzmann said.

The research should also rekindle debate about the risks and benefits of hormone replacement therapy. That is because, in the new study, the higher risk of ovarian cancer was observed only in women who had never taken HRT.

In fact, women who took HRT had a slightly lower risk of developing ovarian cancer, even if they were overweight or obese.

A blockbuster study published in the summer of 2002 showed that HRT, a drug treatment used by millions of women to treat the symptoms of menopause, increased the risk of breast cancer and cardiovascular disease.

Follow-up research has painted a more nuanced picture, showing that when a woman begins HRT matters (immediately at menopause is safer than taking it years later), as does duration of use. There are also distinctions between the use of a combination of estrogen and progestin, as opposed to estrogen alone. Whether a woman has had a hysterectomy also influences the outcome.

An estimated 2,500 Canadian women were diagnosed with ovarian cancer and 1,700 women died from the disease in 2008, according to estimates by the National Cancer Institute of Canada. It is one of the most deadly forms of cancer, with a five-year survival rate of only 37 per cent.

Early detection is difficult because symptoms are vague and there is no simple screening test. Symptoms of ovarian cancer include bloating, abdominal pain, the need to urinate frequently, and a general malaise. Ovarian cancer also tends to occur around menopause, and many women assume their symptoms are related to menopause rather than cancer.

The new study involved a total of 94,525 women aged 50 to 71 who were followed over a period of seven years. Researchers documented 303 cases of ovarian cancer among participants.

A study published last year showed that obese women who were diagnosed with ovarian cancer had a higher risk of recurrence and a higher risk of death.