How do birth control pills prevent pregnancy?

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How does it prevent pregnancy even if a couple does unprotected sex? (I’m aware it’s not 100% effective but still)

In: Biology

The menstrual cycle is controlled by hormones which make sure that a single egg is released and all the processes take place in the right way. What the birth control pills does is that they replace these hormones so that you get the same hormones and a similar menstrual cycle but without an egg. Essentially you are fooling the body into thinking there is already an egg on the way.

Women naturally have a cycle of hormone levels about once every 28 days (varies). Mostly progesterone, estrogen, testosterone, etc. They make the menstrual cycle happen, which can become pregnancy after sex.

There is not just one birth control pill. The various options have different actions, but they all work by managing the sex hormones during the cycle. Some prevent the egg follicles from maturing, some prevent any fertilized egg from attaching to the uterus, some even prevent periods completely.

The point is, they either prevent an egg from being available, or ensure it can’t develop into a fetus.

If they fail, there’s plan B, which will provide such a shock to the hormones that the uterus empties out immediately. Its like the worst period ever.

GYN here- the main effect of birth control pills on the market now is to thicken cervical mucous. Earlier pills in 60s-70s had a higher dose of estrogen (50 micrograms and up) which would prevent ovulation. The newer pills nowadays have way less estrogen (or none at all). The progestins (all of them) thicken cervical mucous and prevent pregnancy by keeping sperm from entering the uterus. Sometimes you won’t ovulate, but that’s not the main method of action anymore. A side effect is thinned endometrial lining which makes periods lighter