Women who are overweight or obese are more likely to have a miscarriage even when the foetus is healthy, according to new research.

While most miscarriages among women of a healthy weight occur because the foetus has genetic faults, most pregnancies lost by heavier women involve normal foetuses that would usually be expected to survive to birth, a study in the United States has revealed.

The findings suggest that the increased risk of miscarriage that affects overweight and obese women occurs not because they are more likely to conceive abnormal embryos, but because their bodies are somehow less capable of carrying a healthy foetus to term.

Several recent studies have indicated that obese and overweight women are more likely to have a spontaneous abortion, most notably a review of the evidence published last month by scientists at the University of Sheffield, which found their risk of miscarriage was increased by 67 per cent.

Inna Landres, of Stanford University School of Medicine in California, who led the new research, said: “The excess miscarriage rate in overweight and obese women is due to the loss of chromosonally normal embryos. This is independent of age and unlike age it’s a modifiable risk factor. Obesity predisposes women to miscarry normal babies.”

About one in four pregnancies ends in a miscarriage, most of which occur in the first three months. Between 50 and 70 per cent of these involve embryos with chromosomal abnormalities known as aneuploidies, which compromise their viability.

Normal embryos have 46 chromosomes – the packets that carry genes – divided into 23 pairs. Aneuploid embryos have either too many or too few, and most fail to implant in the womb or to develop properly. Only rarely do they survive to birth, and those that do suffer disorders such as Down’s syndrome, which is caused by an extra copy of chromosome 21.

The risk of miscarriage rises sharply with increasing maternal age after 35, largely because older women are more likely to conceive aneuploid embryos. Older mothers are also more likely to have children with Down’s syndrome, the most common chromosomal disorder.

In the new research, Dr Landres’s team conducted a genetic analysis of 204 foetuses miscarried in the first trimester of pregnancy, to determine their chromosomal status. They then compared the results with the body mass index (BMI) of the women who had miscarried.

BMI is the standard measure of obesity, calculated by dividing a person’s weight in kilograms by the square of their height in metres. A score of 25 is the threshold for overweight, and 30 for obesity.

The results showed that among women with a BMI of less than 25, 63.4 per cent of the miscarried foetuses were chromosomally abnormal. Among women with a BMI of over 25, only 47.1 per cent of the lost foetuses were aneuploid: the majority were chromosomally normal.

Dr Landres, who presented the study at the American Society for Reproductive Medicine conference in San Francisco, said the results suggested that overweight women trying to start a family could improve their chances of a successful pregnancy by losing weight.

She said: “What can we do to help patients? It’s important to identify elevated BMI as a risk factor for miscarriage and counsel those women who are affected on the importance of lifestyle modification.

“We are trying to figure out what is a preventable loss of pregnancy, whether it is a natural cause or maternal influence. These are all highly desired babies.

This study shows there is maternal influence involved so there’s something we can do about it at the time of conception.”

The effect could be caused by insulin resistance, which often accompanies obesity and can affect a woman’s hormonal balance during pregnancy. Obesity also often raises levels of oestrogen and male hormones, which could also be implicated.

Mark Hamilton, chairman of the British Fertility Society, said: “The association between obesity and miscarriage is well known, and this study suggests that something other than genetic abnormality in the conceptus is implicated. The study certainly confirms the wisdom of advising women contemplating pregnancy that their risk of complications including miscarriage will be reduced if they can achieve a healthy weight.”

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