The most used treatment in the world for emergency contraception is the levonorgestrel (LNG) pill. However, its efficacy decreases if it is administered 3 days after unprotected sexual intercourse, whereas the ulipristal acetate (UPA) pill is effective up until 5 days afterwards. Pooled clinical data show that UPA is more effective than LNG when taken very shortly after intercourse (within 24 h) or, conversely, between 72 and 120 h after intercourse. UPA is also more effective than LNG in inhibiting follicular rupture when administered near the time of ovulation. We show here why overall UPA is more effective than LNG in reducing the rate of unwanted pregnancies by demonstrating the effect of each product depending on the follicular size at the time of an unprotected sexual intercourse We also explain the difference between UPA and LNG in the maximum time to administration simply by the shift in ovulation and the fact that UPA has an effect on larger follicles than LNG does (18 mm vs. 14 mm), without postulating a hypothetical endometrial effect. We also explain why UPA and LNG remain emergency contraceptives and should not be used for daily contraception. (C) 2015 Elsevier Masson SAS. All rights reserved.