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we still ovulate, but (this might get nasty), the mucus on the Uterine lining thickens so that if a egg were to get fertilized, it cannot implant to the uterine wall and develop into an embryo.
Hope that helped a little.
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from
Wiki:
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Several different types of 'the Pill' exist. Generally, they all have different synthetic estrogens and progestins, chemical analogues of the natural hormones, estradiol (an estrogen) and progesterone (a progestagen). Most common brands use 20 to 40 micrograms of ethinylestradiol as the estrogen component and either a fixed or varying (the bi- and triphasic pills) amount of either levonorgestrel or norethindrone as the progestagen component. An exception is the progestin only pill, which lacks estrogen and thus generally has fewer side effects than the combined pill.
The combined Pill primarily prevents pregnancy by preventing ovulation. It also has the side-effect of thickening the mucus over the cervix, which can prevent or slow sperm entry into the uterus. The Pill also thins the endometrium (the lining of the uterus).
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cliff notes: pills that contain estrogen (most) mostly stop ovulation, but the other ingredient progestin makes the uterin wall unreceptive to embryos as a backup safety