Does ulipristal acetate emergency contraception (ella®) interfere with implantation?

Ulipristal acetate (UPA) 30 mg (ella®, HRA-Pharma, Paris, France) acts as an emergency contraceptive (EC) by delaying ovulation. Because it is a selective progesterone receptor modulator, an additional effect on interfering with implantation has been suggested.

This review discusses the evidence for, and against, an anti-implantation effect of UPA-EC.

Primary research on the effect of UPA, at a relevant dose, on endometrium, implantation, efficacy and pregnancy outcome.

UPA-EC does not appear to have a direct effect on the embryo. Changes in endometrial histology are small and not consistent, varying among studies. While UPA-EC affects the profile of gene expression in human endometrium, the findings vary between studies, and it is not clear that these changes affect endometrial receptivity or prevent implantation. UPA at pharmacological concentrations does not appear to have any inhibitory effect on embryo attachment in in vitro systems of human endometrium. UPA-EC is not more effective at preventing pregnancy than chance alone if used after ovulation and does not increase miscarriage rates.

An anti-implantation effect of UPA is highly unlikely at the dose used for EC. Maintaining the warning on the FDA-approved label that "it may also work by preventing implantation to the uterus" might deter some women from using EC, leaving them no option to prevent unwanted pregnancy after unprotected sexual intercourse.