Question

Explain how the abortion pill and emergency contraceptive pills work.

Answer 1

Mifepristone It is a 19-norsteroid with potent antiprogestational and significant antigluco- corticoid, antiandrogenic activity.
Given during the follicular phase, its antipro- gestin action results in attenuation of the midcycle Gn surge from pituitary → slowing of follicular development and delay/failure of ovulation. If given during the luteal phase, it prevents secretory changes by blocking progesterone action on the endometrium. Later in the cycle, it blocks progesterone support to the endometrium, unrestrains PG release from it—this stimulates uterine contractions. Mifepristone also sensitizes the myometrium to PGs and induces menstruation. If implantation has occurred, it blocks decidualization, so that conceptus is dislodged, HCG production falls, secondary luteolysis occurs–endogenous progesterone secretion decrea- ses and cervix is softened. All these effects lead to abortion.   
Mifepristone is a partial agonist and com- petitive antagonist at both A and B forms of PR. In the absence of progesterone (during anovulatory cycles or after menopause) it exerts weak progestational activity—induces predecidual changes. Therefore, it is now regarded as ‘progesterone receptor modulator’ rather than ‘pure antagonist.’ The weak agonistic action is not manifest in the presence of progesterone.

Emergency (postcoital) pill
use in a woman not taking any contraceptive who had a sexual intercourse risking unwanted pregnancy.   
Even if ovulation and fertilization occur, the blastocyst may fail to implant because endometrium is either hyperproliferative or hypersecretory or atrophic and in any case out of phase with fertilization—not suitable for nidation. This action appears to be the most important in case of minipills and postcoital pill.
Uterine and tubal contractions may be modified to disfavour fertilization. This action is uncertain but probably contributes to the efficacy of minipills and postcoital pill.
The postcoital pill may dislodge a just implanted blastocyst or may interfere with fertilization/implantation.