Overweight women are more likely to lose a healthy baby, according to a study. The researchers said the findings supported advice that obese women should lose weight before trying to conceive.

"The excess miscarriage rate in overweight and obese women is due to the loss of chromosomally normal embryos," said Inna Landres, of Stanford University School of Medicine. "It's important to identify elevated BMI [body mass index] as a risk factor for miscarriage and counsel those women who are affected on the importance of lifestyle modification."

Landres' team carried out genetic analyses on 204 miscarriages in women with an average age of 35.

Of the 153 women who had a BMI of less than 25, 36.6% had miscarried foetuses with no chromosome defects, either via insertions or deletions of DNA. This compared with 52.9% of the 51 overweight women (with a BMI of more than 25).

The results were presented at the American Society for Reproductive Medicine annual meeting in San Francisco.

One possible cause is insulin resistance, the early stages of type II diabetes, which affects a woman's hormonal state.

Mark Hamilton, who is chairman of the British Fertility Society but was not involved in the study, said obesity was a recognised cause of miscarriage. He added: "This study will aid our understanding of the known association with being overweight and reproductive loss."