Overweight women are significantly more likely to experience fertility problems, according to a study published in the journal Human Reproduction last week. Obesity is defined in adults as having a body mass index (BMI) above 30. The study found that for every BMI unit above 29, the probability of achieving pregnancy was reduced by four per cent.
Dr Jan Willem van der Steeg of the Academic Medical Centre in Amsterdam, who led the study, told the BBC that the findings were 'worrying' in light of increasing obesity levels in the UK and elsewhere. 'We think that women should be informed about their lower pregnancy chances due to their overweight', he said in a statement, adding: 'We hypothesise that losing weight will increase the chance to conceive without treatment'.
BMI is a measure of body fat based on height and weight. The BMI categories used clinically are normal (18.5-24.9), overweight (25-29.9) and obese (greater than 30). Last month, new guidelines from the British Fertility Society recommended that severely obese women, who are under 37 and therefore not in danger of thwarting their reproductive years, should have their fertility treatment deferred until they have lost weight. However at the time some critics voiced concerns that BMI was not a good indicator of body fat in all women, such as those who have a lot of muscle.
The researchers examined 3,000 'sub-fertile' women - those who have had at least one year of unprotected sex without conceiving - in the first study of its kind to look at the link between BMI and pregnancy chances in a large group of women who have no obvious reasons for infertility. At the top end of the scale, very obese women (with a BMI of over 35) were found to be 26-43 per cent less likely to conceive than women within an average BMI range of 21-29.
One theory is that leptin - a hormone that regulates appetite and energy expenditure and is secreted in fatty acids - may affect hormone levels in obese women. 'It is possible that obese women may have disturbed hormone levels, which decrease the chances of successful fertilisation and implantation', said Dr van der Steeg, who is convinced that rising levels of obesity is a primary factor in the increasing numbers of couples seeking fertility treatment.