Slim women are more likely than morbidly obese women to develop a painful condition that can lead to infertility, a new study has found.

Endometriosis is a condition in which the lining of the womb - the endometrium - grows outside the womb. It can be intensely painful and symptoms include unusually heavy periods and difficulty getting pregnant. It affects around 10% of women.

The study, the largest of its kind to investigate this link, followed the progress of over 5,500 women diagnosed with endometriosis over a 20-year period. The women were aged between 25 and 42 at the beginning of the study.

All provided information about their medical history every two years, including what height and weight they were when they were aged 18.

The US scientists carrying out the study found that overall, women with a body mass index (BMI) of 40 or more - i.e. those who were morbidly obese - had a 39% reduced risk of developing the condition compared to women with a BMI of 18.5 - 22.4. This is the lower end of the ‘normal' range.

Furthermore, when the scientists analysed the women's BMIs when they were 18 years of age, those who were morbidly obese at that age had a 41% reduced risk of developing endometriosis compared to those with a low BMI of 18.

However, the team from Iowa emphasised that while BMI does appear to be linked to endometriosis, the study does not show that having a low BMI actually causes the condition. The reason for the link is unclear.

"Further research is needed to understand the biological mechanisms underlying the associations that we have seen in our study. Maintaining a healthy body weight throughout childhood, adolescence, and adulthood is associated with a myriad of known health benefits.

"The study does not suggest that the morbidly obese women are, in some way, healthier than the lean women and that is the reason for their lower risk of endometriosis. It is more likely that factors related to infertility, which is more common among the very obese, are linked to the reduced risk of endometriosis," the scientists said.

Details of these findings are published in the journal, Human Reproduction.