How Does Plan B Interfere With Ovulation?
Timing in the menstrual cycle determines whether or not Plan B interferes with the ovulatory process. The size of the follicle and when the LH surge occurs are the determinate factors of whether or not ovulation is affected. Plan B taken 24 hours before the LH Serge generally suppresses the surge completely in which ovulation does not occur. When the dominate follicle reaches 18 to 20 mm is size the incidence of pregnancy is rather high due to the fact that ovulation still occurs in a high percentage of those patients and follicular rupture is only delayed in a small percentage of those patients who take Plan B. When Plan B (morning after pill, day after pill, post coital contraception, day after contraception) is used closer to the LH surge, it can delay or reduce the peak of the LH surge, or may not affect the surge at all. Ova released after a delayed or partially depressed LH surge are resistant to fertilization.
