Medical abortion methods are especially effective in the first trimester, most effective before 49 days of pregnancy. This is due to the fact that both Mifepristone and Methotrexate cause detachment of the trophoblast (outer layer of developing embryo which ultimately forms the placenta) from the uterine wall, (13) which is easier to accomplish the less developed the pregnancy.

 

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Mifepristone (aka RU486) is more effective the earlier in pregnancy it is used: "women with pregnancy durations of 7 weeks or less LMP experience a complete abortion about 95% of the time. Success rates decrease to about 80% in the ninth week LMP."(5) Mifepristone acts primarily as an anti-progestational agent.(7) The progesterone receptors in the endometrium are blocked, and the disruption of the embryo and trophoblast lead to a decrease in human chorionic gonadotropin (HCG) produced by the placental tissues and therefore a withdrawal of hormonal support for the corpus luteum. This provides the rationale for why this method is more effective the earlier the pregnancy, because the more dependent the pregnancy is on progesterone produced by the corpus luteum, the more likely the action of the progesterone antagonist, mifepristone, will result in abortion.(7) (See Female Reproductive Cycle) The regimen involves three visits to an abortion provider. On the first visit, the woman takes 600 mg mifepristone orally. She returns two days later for administration of Misoprostol, a prostaglandin which serves to soften and open the cervix and cause contractions of the uterus, expelling the contents of the pregnancy. In 5% of pregnancy, mifepristone alone is enough to induce abortion, but in all other cases 400-800 ug of misoprostol is administered orally or vaginally.(5) Following administration of misoprostol, "three-fourths experience expulsion within 24 hours…and 80-95% abort within 2 weeks."(6) The third visit is for follow up.

 

Methotrexate and Misoprostol

Methotrexate, which is not approved as an abortifacient but is approved for other indications, has been used by physicians in the United States to induce abortion, especially in light of the struggle surrounding the approval of Mifepristone.(13) It is a cytotoxic drug that blocks folic acid in fetal cells, inhibiting division and growth of trophoblastic tissue.(6) Cell division of the developing embryo is halted, so development of the pregnancy cannot proceed. Several Methotrexate-misoprostol regimen protocols are used, but all involve first the injection of 50 mg methotrexate per square meter of body surface into the gluteal muscle. 800 ug misoprostol is administered vaginally 3 to 7 days later, often in the privacy of a woman's own home.(5) Follow up visits are scheduled to ensure completion of the abortion and safe recovery. Methotrexate combined with Misoprostol is about 95% successful in terminating early pregnancies.(5)