Contraceptive pills. A variety of oral contraceptive pills used for birth control by women. These pills contain synthetic female sex hormones that mimic the effects of oestrogen and progesterone in the body. All the pills contain a progestogen drug, which suppresses ovulation, makes cervical mucus impenetrable to sperm and inhibits implantation of fertilized eggs in the uterus. An oestrogen drug, which also suppresses ovulation, may be combined with the progestogen. The probability of a pregnancy for each year of use is 0.5% for combined oral contraceptives and 2.5% for progestogen-only pills. Progestogen-only pills have fewer side-effects.