Overweight women are more likely to miscarry a healthy baby, according to research involving 204 women who had suffered a miscarriage.

The researchers said the findings back up 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 Dr 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 of 204 foetuses miscarried by women with an average age of 35.

Of the 153 women with normal body weight (a BMI of less than 25), 36.6% had miscarried foetuses with no chromosome defects – insertions or deletions of DNA. This compared with 52.9% of the 51 overweight women (BMI over 25).

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

One possible cause of the effect is insulin resistance, which is the precursor of type 2 diabetes. This affects a woman's hormonal state, which could impact the pregnancy. Also, excess adipose (fatty) tissue leads to higher levels of oestrogen and testosterone in the body.

'We are trying to figure out what is a preventable loss of pregnancy, whether it is a natural cause or maternal influence," said co-author Dr Ruth Lathi.

Dr Mark Hamilton at the University of Aberdeen, who chairs the British Fertility Society and was not involved in the study, said obesity is a recognized cause of miscarriage. "It has not been defined if that risk is related to genetic problems for the embryos or the obesity itself is linked to implantation mechanisms," he said.

"This study will aid our understanding of the known association with being overweight and reproductive loss. We need more follow-up studies on this."