Hormonal contraceptives contain a progestin (progesterone medicine) with or without an estrogen. Both progestin and estrogen are made in a laboratory and are similar to the hormones that all women naturally produce. These two hormones together, or the progestin alone, work in several ways to prevent a pregnancy: they can prevent ovulation (the release of an egg), they make the mucus around the cervix (mouth of the womb) thicker so that sperm cannot enter the uterus (womb), and they make the lining of the uterus (womb) thinner to prevent a fertilized egg from attaching itself.
You can find more information from the American Society for Reproductive Medicine on this topic here:
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