All methods of medication abortion have an associated failure rate.
A medication abortion is considered to have failed if 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 methortrexate 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 mcg inserted vaginally followed
by a repeat 800 mcg vaginal dose 24 hours later) 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 a
third 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.
