By Emma Wilkinson
BBC News health reporter in Amsterdam
Obesity cuts the chances of getting pregnant
Obese women who lose even a small amount of weight may boost their chances of getting pregnant, a study suggests.
Researchers carried out a three-month study of 40 obese women who were not ovulating.
They found a 5% loss in body weight was linked with a 19% increase in blood flow to the womb.
Improved circulation can help trigger the egg release from the ovaries and may help an embryo implant in the womb.
However, the researchers from the University of Sheffield said more work was needed to determine how the finding translates to actual pregnancy rates.
IVF on the NHS is restricted to those with a body mass index (BMI) of 30 or under and the British Fertility Society recommends a cut off of 35 because of the risks to both mother and baby.
Women in the study, most of whom had polycystic ovary syndrome (PCOS), were around 29 years old on average and had a body mass index of almost 40, delegates at the European Society for Human Reproduction and Embryology (ESHRE) heard.
They were prescribed weight loss drugs orlistat or metformin and managed to lose 5% of their body weight in 12 weeks.
Previous work by the team had shown small amounts of weight loss were related to a 30-40% chance of ovulation returning in these patients.
But this is the first study looking at how blood flow is involved.
The researchers said that the increased blood flow acted as a "switch" to kick start the ovary.
Levels of the male hormone testosterone - high concentrations of which are found in PCOS patients - also fell as blood flow improved, which would also boost a woman's chances of conceiving naturally.
The findings also raise the possibility for looking into whether a drug that improves the flow of blood to the womb could help women get pregnant.
Women with PCOS, which is one of the most common causes of infertility, tend to put on weight because of their condition and struggle more than other women to lose it through diet and exercise.
Professor Bill Ledger, from the University of Sheffield, said asking women to lose 5-10% was a "modest target".
"The message for women with PCOS is don't think you have to lose half your body weight.
"This could also encourage moderately overweight women to lose 5-10%."
Dr Daghni Rajasingam, spokeswoman for the Royal College of Obstetricians and Gynaecologists said the findings were interesting but the key was whether the improved blood flow ended up with a successful pregnancy.
She stressed that infertility was just one of problems which obese women who wanted a child faced.
Even if they succeeded in getting pregnant, their risk of complications was much higher, and their chances of getting to full term lower.
She said: "Being as close as possible to your ideal body weight would be a good thing for all aspects of pregnancy."