Mechanism of action of emergency contraceptive pills.

Ulipristal Acetate (UPA) One study has demonstrated that UPA can delay ovulation. In this study, 34 women were treated when the size of the leading follicle was at least 18 mm. Each woman contributed one cycle treated with placebo and another with UPA. Follicular rupture failed to occur within 5 days following UPA treatment in 20 (59%) subjects while normal ovulation was observed in all women within 5 days after placebo intake. Follicular rupture failed to occur within 5 days after treatment with UPA in all women treated before onset of the LH surge, in 79% of women treated after the onset of the LH surge but before the LH peak, and in 8% of women treated after the LH peak. Another study found that UPA altered the endometrium, but whether this change would inhibit implantation is unknown.