Effect of levonorgestrel emergency contraception on implantation and fertility: A review

Levonorgestrel-only emergency contraception (EC) inhibits ovulation to prevent fertilization. Misconceptions regarding its mechanism of action contribute to low use in some settings. We aimed to review the mechanism of action of Levonorgestrel EC and assess the evidence for a postovulatory effect on viable pregnancy development in the exposed and subsequent cycles.We searched Pubmed, Embase, Web of Science, clinicaltrials.gov, the Cochrane database, and the US FDA Adverse Event Reporting System. We included studies reporting on the effect of Levonorgestrel EC exposure on (1) ovulation, (2) conception, (3) implantation, (4) ectopic pregnancy, (5) pregnancy viability, and (6) fertility in subsequent cycles. Data were extracted and tabulated from included studies by two researchers. The results were analyzed and summarized in narrative form.We included 33 studies. Women exposed to Levonorgestrel EC at or after the LH (luteinizing hormone) surge had similar rates of inhibited ovulation (3 studies) and conception rates (2 studies) as controls. Nine out of ten studies found no difference in endometrial receptivity to implantation after exposure to Levonorgestrel EC compared to controls. The evidence for an association between Levonorgestrel EC and reduced fallopian tube motility or ectopic pregnancy was conflicting. We found no evidence of increased miscarriage or teratogenicity (3 studies) or disrupted menstrual cycle (3 studies) after Levonorgestrel EC exposure compared to controls.This review supports that Levonorgestrel EC taken after ovulation does not affect implantation and results in similar conception rates compared to placebo. There is no evidence that exposure to Levonorgestrel EC affects fetal development, miscarriage, stillbirth, or subsequent menstruations.