Obese women who want to become mothers are told to lose weight to increase their chances of having a baby.
Now new research suggests slimming down helps not because it boosts ovulation, but simply by increasing a woman's sexual appetite.
A team from Penn State College of Medicine studied how weight-loss surgery affected reproductive function in a group of morbidly obese women.
Study leader Dr Richard Legro, said: 'Obesity in women has been linked to lack of ovulation and thus infertility.
'Obesity, especially centered in the abdomen, among infertile women seeking pregnancy is also associated with poor response to ovulation induction and with decreased pregnancy rates.'
The team took urine samples to measure ovarian hormones over the course of a menstrual cycle.
The quality of the ovulation also remained unaltered. The only change seen was a shortening of the first half of the menstrual cycle, from the end of the previous menstrual flow until the release of the egg
However, questionnaires filled in by the participants revealed losing the pounds had a dramatic impact on libido, with large increases in sexual desire and arousal. The researchers said this may have led the women to have sex more often.
'The effects of weight loss on reproductive function are more modest than we hypothesized. In terms of ovulation, there doesn't appear to be a window after surgery where fertility is improved,' Dr Legro said.
'The door appears to be open at all times. Other factors may be involved with infertility in obese women, such as diminished sexual desire and thus less intercourse.'
The findings were reported in the Journal of Clinical Endocrinology and Metabolism.
It follows a recent study that found losing weight could boost sexual desire in obese men.
The research, published in The Journal of Sexual Medicine, found losing five per cent of body weight boosted the men's testosterone levels and helped them last for longer during sex.