In the UK over half of women of reproductive age are either overweight or obese. Obesity has an adverse effect on natural fertility - obese women are three times more likely to be infertile and have a higher risk of pregnancy complications, including miscarriage. However, the impact of raised body mass index (BMI) on the miscarriage rate after IVF is not clear, and previous studies have been inconsistent.
Consultant Mr Tarek El-Toukhy led a study of more than 400 women undergoing IVF at the Assisted Conception Unit at Guy's and St Thomas' NHS Foundation Trust between 2006 and 2010. All the women underwent single blastocyst transfer which is associated with a higher pregnancy rate and lower miscarriage rate compared with single cleavage embryo transfer.
Overall, 27 percent (%) of the women miscarried before 23 weeks into the pregnancy. Women with a BMI higher than or equal to 25 had more than double the risk of miscarriage compared with women who had normal BMI: 38% versus 20% of the women miscarried. When the scientists subdivided the women into overweight and obese, they found these women had comparable miscarriage rates: 37% and 42% respectively.
El-Toukhy said: 'We were amazed at how large an impact being overweight – not necessarily obese – has on the success of fertility treatment. To maximise the chance of a successful pregnancy, we are now recommending that women get as close as possible to a healthy weight before starting treatment'. The authors suggest that altered hormone levels of insulin and leptin in overweight women might be to blame as these affect the endometrium (the lining of the womb) and implantation of the embryo.
The UK's National Institute of Health and Clinical Excellence states that women with a BMI over 30 who opt for IVF should be warned of potential difficulties. But El-Toukhy goes further than this and said: 'We shouldn't stop at 30, we should aim for a BMI as close as possible to the healthy range'.
'It confirms what we thought we knew', said Mr Stuart Lavery, a gynaecologist at the Imperial College NHS Trust in London. 'Now there's really good justification to employ weight loss programmes as part of fertility management'.
Dr Allan Pacey, a fertility specialist at the University of Sheffield said: 'The effect of obesity on the outcome of assisted conception has been unclear. This is an excellent study which clearly demonstrates that obesity is an important factor in the miscarriage risk after IVF. It highlights the need for women, and I would argue couples, to try and achieve a healthy weight prior to IVF'.
The study was published in the journal Human Reproduction.