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Contraceptive pills. A packet of combined oral contraceptive pills used for birth control by women. The packet contains 21 pills, taken daily for the three week period between menstruations. For maximum effect each oral pill should be taken at about the same time each day. These pills contain two synthetic female sex hormones: a progestogen and an oestrogen. The progestogen suppresses ovulation, makes cervical mucus impenetrable to sperm and inhibits implantation of fertilized eggs in the uterus. The oestrogen suppresses ovulation. The probability of a pregnancy for each year of use is 0.5%.