They found women who used complementary therapies while undergoing IVF were 20% less likely to fall pregnant than those who used IVF alone.
The Cardiff University team believe herbal remedies could possibly interfere with IVF drugs, but experts at a fertility conference in Lyon said stress was probably key.
Indeed, the women who turned to complementary and alternative medicine (CAM) had been trying fertility treatments for longer and reported being more stressed by their fertility problems than the other women in the study.
The researchers examined the psychological and medical profiles of 818 women at the start of their IVF treatment and which of them went on to use CAM in the subsequent 12 months.
Overall, 261 or 32% of the women used some form of CAM and usually more than one type. Nearly half of these had used reflexology and over a third had used nutritional supplements.
Women in the CAM group had more attempts at IVF – three compared to two attempts in the non-CAM group, on average – but were less likely to become pregnant.
Lead researcher Dr Jacky Boivin said repeated failure with fertility treatment might prompt women to use CAMs. “But it may be that complementary therapies diminish the effectiveness of medical interventions. Perhaps women should hold off until they have tried conventional fertility treatments.”
Mr Michael Dooley, consultant gynaecologist at the Poundbury Clinic in the UK, said: “It is difficult to draw conclusions from this study because it is not comparing like with like. It’s well recognised that some CAMs can reduce stress. And stress can have an impact on fertility.”