Women with obesity who participate in weight loss programmes before starting in vitro fertilisation (IVF) are 47% more likely to conceive naturally than those receiving minimal or no weight loss support, according to the most comprehensive systematic review of the evidence to date.
The University of Oxford-led research, funded by the National Institute for Health and Care Research, analysed 12 international trials involving 1,921 women. Beyond the increase in natural conception, the study found weight loss interventions increased the odds of any pregnancy (naturally or by IVF) by 21%.
The findings, published in the Annals of Internal Medicine , are particularly significant as NHS commissioning bodies across England restrict access to NHS-funded IVF for women with a Body Mass Index (BMI) over 30. The study suggests that providing structured weight loss support could not only help women become eligible for NHS-funded treatment but may also increase their chances of conceiving without medical assistance.
Dr Moscho Michalopoulou , lead author from Oxford's Nuffield Department of Primary Care Health Sciences , said: 'Currently, women with obesity can face a double challenge with higher rates of infertility and potential exclusion from NHS-funded IVF. Our findings offer hope. They suggest that offering structured weight loss support could improve the chance of conceiving naturally, which may avoid the need for IVF treatment. Programmes that help women achieve greater weight losses have the potential to help more women achieve a successful outcome, and should be tested in larger high-quality trials.'
The review synthesised evidence from a wide range of interventions, including diet programmes, exercise, and weight loss medications such as orlistat (which reduces fat absorption) and older GLP-1 agonists - earlier versions of the drug class that now includes Ozempic and Wegovy. On average, women in the intervention groups lost 4kg more than those in the comparison groups.
While the review provides the clearest picture yet of the benefits for conception, the researchers stress that the effect on the ultimate goal - a healthy baby - is still unclear.
Associate Professor Nerys Astbury , who led the research at the University of Oxford, noted: 'The evidence on live birth rates was of very low certainty because many studies did not follow the pregnancies through to birth and report this outcome - which is so important to the patients. While there was no evidence of an increase in pregnancy loss, we need larger, high-quality trials to confirm the effect on live births. It was also interesting that when we restricted the analysis to low-energy diet replacement programmes, which lead to greater weight loss, there was an increase in live births, but this needs to be confirmed in larger trials.'
The researchers observed that in studies with more women with Polycystic Ovary Syndrome (PCOS) - a common condition linked to both obesity and infertility - there appeared to be a trend toward smaller benefits for natural conception, though they emphasise this was an exploratory observation requiring further investigation.
The findings raise important questions for healthcare policy.
'This isn't just about improving pregnancy rates - it's about improving equity of access to fertility treatment,' Professor Astbury added. 'We know that people from more deprived areas and those from certain ethnic backgrounds are more likely to live with obesity. Policymakers should consider whether integrating structured weight loss support into fertility services could improve outcomes for patients, while potentially reducing overall costs by increasing the number of women who conceive naturally.'
The team is calling for larger, more robust trials that compare specific weight loss programmes and follow all participants through to birth to determine which interventions are most effective and for whom.
Dr Michalopoulou concluded: 'For women trying to conceive, every month matters. We need efficient, evidence-based pathways that give them the best chance of success. Our work suggests that for many women with obesity, an intensive supported weight loss programme could help them to conceive naturally or make it possible to access IVF services.'
Read the full paper, ' The Effect of Weight Loss Before In Vitro Fertilization on Reproductive Outcomes in Women With Obesity: A Systematic Review and Meta-analysis ' in Annals of Internal Medicine .
