Obesity cuts the chance of IVF working and increases the risk of premature birth and stillbirth, the largest US study into the issue has found.

The effect becomes more profound as weight increases, with the most obese women having the lowest chance of success, according to data from almost 50,000 women.

The most obese women in the study had 35 per cent less chance of falling pregnant and a 59 per cent increased chance of giving birth to a very premature baby.

But overweight women were also at risk, with a 13 per cent lower chance of giving birth to a live baby and a 16 per cent increased risk of premature birth.

Barbara Luke, professor of obstetrics and gynaecology at Michigan State University in the US, presented her findings at the American Society for Reproductive Medicine conference in Atlanta.

She said: "The take-home message from this is that women need to reduce their weight before trying fertility treatment.

"Obesity increases the chance of having a premature baby and reduces the odds of pregnancy. And it increases the chances of stillbirth. 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."

The study included 28,094 women of normal weight, with a body mass index (BMI) of 18.5 to 24.9. Another 11,710 women were overweight (BMI of 25 to 29.9), 5,187 were class I obese (BMI of 30 to 34.9), 2,276 were class II obese (BMI of 35 to 39.9) and 1,415 were class III obese (BMI over 40).

The results revealed that class I obese women had a nine per cent lower chance of falling pregnant, a 20 per cent lower chance of a live birth and more than double the risk of stillbirth. They were also 33 per cent more likely to have a premature birth.

Among class II women, they were 28 per cent less likely to fall pregnant, 26 per cent less likely to have a live birth, had more than double the risk of stillbirth and a 38 per cent increased risk of premature birth. And class III women were 35 per cent less likely to get pregnant, 25 per cent less likely to have a live birth, had more than double the risk of stillbirth and were 34 per cent more likely to give birth prematurely.