Combined contraceptive pills work in numerous ways to prevent pregnancy occurring. Firstly, they work to stop ovulation, which is when the ovaries release an egg each month. Without an egg to fertilise for the sperm, pregnancy cannot happen. Combined pills also work to thicken the mucus in the womb, thereby making it harder for sperm to pass through the womb to reach the egg. They also act by thinning the lining of the womb, thereby reducing the chance of a fertilised egg implanting into the womb and being able to grow.
Progesterone only pills work in a slightly different way to stop pregnancy occurring. Mainly they act by thickening the mucus in the cervix to stop sperm from reaching an egg. Desogestrel (Cerazette), which is a particular type of progestogen-only pill can also work to stop ovulation.
