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Contraceptive pills. Blisterpack of oral contraceptive pills used for birth control. These pills contain synthetic female sex hormones which interfere with the normal menstrual cycle and prevent unwanted pregnancies. The hormones ensure that no egg is released from the ovary during the menstrual cycle. They also cause the fluid in the neck of the uterus to thicken, making it more difficult for sperm to enter. Finally, they prevent the thickening of the lining of the uterus in preparation for the implantation of a fertilised egg.