Women who are overweight or obese have lower chances of successful IVF treatment, according to researchers reporting at the American Society for Reproductive Medicine (ASRM) conference in Atlanta, US, this week. The researchers, from Michigan State University in the US, found that women who were defined as clinically obese were up to 35 per cent less likely to conceive and have a live baby, and twice as likely to have a stillbirth, than their lighter counterparts.

Women who were classed as overweight were 13 per cent less likely to conceive and were 16 per cent more likely to have a premature birth than those of normal weight. The study involved nearly 50,000 women who were grouped according to their body mass index (BMI), which is calculated as weight in kilograms divided by height in meters squared. Those with a BMI between 20 and 25 were considered to be normal weight, those between 25 and 30, overweight, and those above 30, obese.

Dr Barbara Luke, who led the study, said that 'the take-home message from this is that women need to reduce their weight before trying fertility treatment'. She added: 'Obesity is a state of inflammation and increasing obesity is not good. It is not conducive to conception and it is not conducive to pregnancy'. Women in the study with a BMI between 30 and 34.9 (5187) had nine per cent less chance of getting pregnant, a 20 per cent lower chance of a live birth, were on average 33 per cent more likely to have a premature birth and had more than double the risk of a stillbirth. Tam Fry, from the National Obesity Forum, said that 'this study underlines the fact that if you are thinking about having children, you should start thinking about losing weight at least six months before you conceive.'

More than one in four Britons is now thought to be obese and that figure rises year by year. Around 3.5 million people in Britain experience fertility problems at some stage, and most National Health Service (NHS) hospitals will not fund fertility treatment for women with a BMI of 30 or more. The British Fertility Society recommended two years ago that women with a BMI over 35 should not receive fertility treatment until they had lost weight.

A recent Australian survey showed that women who are overweight or obese often do not realise they are. In a study of 412 women, 30 per cent of whom were overweight or obese, only 36 per cent said they believed their weight to be normal, and only 16 per cent acknowledged that they were obese.

The results of a Scottish study, reported in BioNews in January, contradict the US researchers findings. It showed that obese and overweight women had the same chances of successful IVF treatment as normal weight women. The only difference noted in that study, similar to the current research, was the higher rate of miscarriage in heavier women.