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Contraceptive pills. Packets of oral contraceptive pills used for birth control. Here, different pills are taken at different times of the menstrual cycle. The days on which to take each pill are seen on the silver pill packs. This is to ensure that the correct daily dose is taken. Contraceptive pills contain one or more synthetic female sex hormones, most containing oestrogen and progesterone combined. Progesterone acts on the pituitary gland affecting the menstrual cycle; oestrogen blocks the monthly release of a mature egg (ovulation). The pregnancy rate when using the contraceptive pill is on average less than 1 per 100 woman years.