From a community pharmacist : oral contraceptives

Abstract

The most widely used type of oral contraceptives is the combined estrogen-progestogen product. Table 1 gives an indication of the range of combination products available today. The combination contraceptive products have a high efficacy resulting from 3 effects: 1. the suppression of ovulation by estrogen and progesetagen; 2. the effect of the progestogen on the endometrium in such a way that it cannot readily support the nidation and growth of a fertilised ovum; 3. the thickening effect of the progestogen on the cervical mucus which prevents the sperm from passing through.