LONDON (Reuters) - Obese women have four times as many unplanned pregnancies as healthy-weight women despite having less sex, and obese men are more likely to have sexual diseases despite fewer partners, scientists said on Wednesday.

In a study showing how obesity can harm sexual health, French and British researchers also found that obese women are less likely to ask for contraceptive advice or use the pill, and obese men are more likely to suffer from erectile dysfunction.

With obesity epidemics overwhelming many wealthy nations and threatening increasing numbers of developing countries, experts said the public health impact of the findings was important.

In the United States, for example, two-thirds of adults are overweight or obese.

“In public health terms, the study lends a new slant to a familiar message: that obesity can harm not only health and longevity, but your sex life,” Sandy Goldbeck-Wood, a specialist in psychosexual medicine at Britain’s Ipswich Hospital, wrote in an editorial on the study in the British Medical Journal (BMJ).

The research, led by Professor Nathalie Bajos of France’s National Institute for Health and Medical Research (INSERM), is the first major study to investigate the impact of being overweight or obese on sexual activity and other factors such as sexual satisfaction, unintended pregnancy and abortion.

The authors surveyed the sexual behavior of 12,364 men and women aged between 18 and 69 years in France in 2006. Around half of them were normal weight, with a body mass index (BMI) of 18.5 to 25, around 2,500 of them were overweight, with a BMI of 25 to 30, and around 750 of them were obese, with a BMI over 30.

Compared to normal weight men, obese men were 70 percent less likely to have had more than one sexual partner in the past year and two and half times more likely to suffer from erectile dysfunction. Obese men under 30 years old were far more likely to have had a sexually transmitted disease.

Sexual dysfunction was not linked to BMI in women, but obese women under 30 years old were less likely than women of normal weight to seek contraceptive advice or use oral contraceptives and were also more likely to report an unplanned pregnancy.

The study also found that obese women were five times as likely to have met their partner on the internet, more likely to have an obese partner, and less likely to view sex as important for personal life balance.

Bajos said social pressure, low self-esteem and concerns about body image may help explain these findings.

Goldbeck-Wood said there was evidence that doctors find it difficult to discuss sex and weight issues with patients, but she said they must be more prepared to do so: “We need to understand more about how obese people feel about their sex lives, and what drives the observed behaviors and attitudes.”