Background: Medical abortion with mifepristone and misoprostol is highly effective; however, continuing pregnancy can still occur, especially as gestation advances. Even after completing treatment, some women with continuing pregnancies change their mind. Legislators began to focus on women who change their mind after mifepristone administration following a 2012 report of “medical abortion reversal” in which women received high-dose progesterone to antagonize mifepristone effects. A recently published case series with 547 women reported continuing pregnancy rates in women receiving various progesterone regimens within 72 hours of taking mifepristone; the highest rates occurred in women receiving high-dose oral progesterone (68%) and progesterone intramuscular (64%). Notably, overall efficacy was higher with more advanced gestation, ranging from 25% at 5 weeks to 77% at 9 weeks. While researchers and legislators debate the concept, no randomized, placebo-controlled trial has evaluated the efficacy of any regimen.

Methods: We propose such a study which will enroll 40 women seeking surgical abortion to receive mifepristone 200 mg then randomized to 2 weeks of oral progesterone therapy or placebo starting the following day. Participants will attend follow-up visits 3 and 7 days after initiating progesterone/placebo and have a surgical abortion, if still pregnant, after approximately 2 weeks.

Proposed analysis: Progesterone, serum hCG and vaginal ultrasound evaluations will occur at baseline and at each follow-up visit. The primary outcome is continuing pregnancy two weeks after mifepristone administration. The sample size is based on loss rates quoted with no treatment and with progesterone by physicians advocating that mifepristone effects can be antagonized with high-dose progesterone.