All methods of medication abortion have an associated failure rate. A medication abortion is considered to have failed if the pregnancy is ongoing and/or an aspiration intervention is required to complete the abortion. Reasons for aspiration intervention include prolonged or excessive bleeding, incomplete abortion (remnants of fetal tissue in the uterus), or an ongoing pregnancy.
For the mifepristone and misoprostol regimen and methotrexate and misoprostol regimen, ongoing pregnancy occurs in less than 1% of cases. However, aspiration intervention (and therefore medication abortion failure) is required for 2%-5% of users of mifepristone and misoprostol and approximately 5% of users of methotrexate and misoprostol. For both of these regimens, the rate of aspiration intervention increases with increased gestational age.
In the high doses used in the chemotherapy regimen, methotrexate exposure during pregnancy has been associated with numerous fetal malformations. Several case reports indicate that methotrexate may have teratogenic effects in cases of incomplete abortion. Women electing to use the methotrexate and misoprostol regimen should be informed of the teratogenic effects of methotrexate and should be counseled on the importance of aspiration completion in the event that the medication abortion is unsuccessful.
The misoprostol alone regimen has a significantly higher failure rate that the other medication abortion methods. When the evidence-based regimen (800 micrograms administered buccally, sublingually, or vaginally repeated after 3-12 hours and repeated again after 3-12 hours for a total of three doses) is used in the first nine weeks of pregnancy, approximately 75%-85% of women will have a successful abortion. In the 15%-25% of cases when the regimen fails, women may require an additional intervention to complete the abortion. This may include administration of an additional dose of misoprostol or an aspiration intervention. Recent studies suggest that the ongoing pregnancy rate after misoprostol administration is approximately 10%. Fetal exposure to misoprostol in utero may increase the risk of a set of limb and central nervous system abnormalities if the pregnancy is carried to term. It is recommended that women who experience a continued pregnancy after taking misoprostol for early pregnancy termination be offered an aspiration abortion.
