Researchers studied women receiving IVF treatment using a donor egg in Spain, and found that those with a BMI of over 30 were 23 percent less likely to become pregnant - and 27 percent less likely to have a baby - than those with a BMI in the range of 20-25. Women who were classified as merely 'overweight', or who were classified 'underweight', were just as likely to have a baby as those with normal BMI.
'Based on our results, the chance of having a baby by egg donation is reduced by around one-third for obese women', said Dr José Bellver, from the Instituto Valenciana de Infertilidad in Spain, who led the study.
Because all of the women had received an egg from a young donor with a normal BMI, the team could attribute embryo implantation failure and higher risk of miscarriage to the womb environment.
'Up until now, it was believed that the effect of obesity in women was only on the [egg cell] or the embryo, but not in the uterus', said Dr Bellver. 'What we have seen is that when you put a good embryo from a donor into the uterus of these women, you have a reduction in implantation rates and the development of these embryos is affected. There is a trend towards higher miscarriage rates and more complications in pregnancy, and at the end, they have almost one-third less chance of having a newborn.'
The team believe that embryos may be less likely to attach and develop normally because of hormonal changes in obese women. 'Insulin resistance, excess of androgens, and hormones that regulate hunger, for example leptin, affect the ovaries and the uterus', said Dr Bellver. 'The fetus is developing in an abnormal environment.'
Dr Bellver advises that women with a BMI over 30 should be encouraged to lose weight with 'multidisciplinary lifestyle therapy' - a combination of diet, exercise and psychological advice. 'Any obese women should lose weight before getting pregnant - no matter how their pregnancy is achieved', he said.
Dr Sue Avery, director of Birmingham Women's Fertility Centre, commented: 'What makes this study so interesting is the fact that it looks at implantation in isolation. There have been similar studies before which have demonstrated an issue with implantation, but this is a huge study, and therefore more definitive.'
She added: 'The more evidence we can present to patients regarding their need to lose weight, the better chance we have of convincing them that this is not just an issue of general health, but has a real impact on their ability to conceive that cannot be rationalised away, and further supports the argument that National Institute for Health and Care Excellence (NICE) guidance does not simply represent a bias against the obese.'
The results were presented at the annual meeting of the European Society of Human Reproduction and Embryology.