Association of the body mass index of women undergoing in vitro fertilisation, with pregnancy, live birth, and pregnancy loss rates, and number of eggs collected: a population-based study of the Human Fertilisation and Embryology Authority database

Lay summary

Individuals with higher body weight have an increased risk of infertility. Numerous studies have reported lower success and higher risk of complications during fertility treatment, such as in vitro fertilisation (IVF), in women with higher body mass index (BMI), calculated using weight and height values. These studies have informed UK clinical guidelines which currently state that women with higher BMI should be advised to lose weight before IVF to increase their chances of getting pregnant. NHS fertility clinics have also set upper BMI cut-offs for accessing IVF. As a result, many women are unable to access IVF in the NHS because of their high BMI. This may lead to inequalities in access to IVF since BMI increases with age, and women from certain ethnic groups are more likely to have higher BMI. However, most studies have analysed a limited number of treatments from single fertility centres. No study has investigated the relationship between BMI and IVF outcomes using a large number of women representative of the UK population.

This project will analyse whether and how the BMI of women undergoing IVF treatment in the UK is related to treatment outcomes, using data from the UK Human Fertilisation and Embryology Authority (HFEA) register between January 2022 and December 2023. The research group will present results separately by whether the IVF treatment was NHS- or privately-funded, and will investigate differences by age and ethnic group. The influence of these factors on the relationship of BMI with IVF outcomes is yet unclear.

Using data from the HFEA Register, this research project aims to clarify the association of BMI with IVF outcomes, and develop the evidence base on which to make decisions that achieve good and equitable outcomes in fertility treatment for all.

Public benefit statement

Women with higher BMI are often unable to access IVF through the NHS. Many women cannot afford private support for effective weight loss nor private IVF care where BMI cut-offs are not always applied. Particular groups of women, such as those of older age, or those belonging to certain ethnicities, may be more likely to have higher BMI. These factors create inequalities in the access to IVF, and it is not clear whether this is justified by data on treatment outcomes. This large analysis of all IVF treatments across the UK will help study the link between BMI and IVF outcomes, assess whether important factors such as age and ethnicity influence this link, and if so, which age and ethnic groups might be more at disadvantage. It will also clarify whether evidence supports the use of BMI cut-offs to access IVF, and aid decision making in the treatment of women who need IVF.