Overweight women are at greater risk of miscarrying a genetically normal baby in the early stages of pregnancy than women who maintain a healthy weight, according to a new study by scientists at the Stanford University School of Medicine in California, US. The researchers, presenting at the annual American Society for Reproductive Medicine (ASRM) conference, suggested that this indicates that a mother's weight can affect the outcome of an otherwise healthy pregnancy.
The UK's NHS estimates that around a quarter of all pregnancies in the UK end in miscarriage. The majority of these occur in the first 12 weeks of pregnancy, but can occur up to 24 weeks. The cause of miscarriage is not always known, but it is thought that between 50 and 70 per cent occur as a result of chromosomal abnormalities (genetic defects) in the fetus. The California researchers tested DNA from 204 fetuses miscarried in the first eight weeks of pregnancy. They compared the rate of chromosomal abnormalities in the fetuses from women with a normal body mass index (BMI) with the rate of abnormalities in fetuses from women with a BMI that classified them as overweight or obese. They found that 53 per cent of babies lost by overweight women had no chromosomal abnormalities compared to just 37 per cent of babies lost by women of a more healthy BMI.
Dr Inna Landres, who led the research team, said that these findings indicate that 'obesity predisposes women to miscarry normal babies.' The reason for this is not yet understood, but Dr Landres suggested that one explanation could be altered levels of hormones such as oestrogen and androgens seen in overweight women. She emphasised: '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.'
An individual's BMI is calculated by dividing their weight in kilograms by their height in metres squared. A BMI of 18 to 25 is considered normal, whilst over 25 is classed as overweight and over 30 as obese. All the women in the current study were attending an academic centre for fertility counselling and had their BMI calculated before conception.
Dr Mark Hamilton, chairman of the British Fertility Society (BFS), said: 'It is recognised that women who are overweight are at a greater risk 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. This study will aid our understanding of the known association with being overweight and reproductive loss.'