So from what I recall, hormonal contraception thickens a woman’s cervix and increases the cervical mucus (what an awful word), which stops sperm entering the uterus. What I’m unsure about is, what happens to the mucus and what happens to the uterine lining for the people whos periods stop?
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The answers you’ve been getting so far aren’t really explaining the process. The endocrine system has an intricate hormonal interplay that is needed for normal ovarian functioning. Hormones in pills interfere with they process. The pill doesn’t “trick your body into thinking you’re pregnant”—that’s a quick explanation the creators of the pill came up with in 1960 to explain this exciting new pill to people who didn’t necessarily have much of an understanding about the female reproductive system or hormones. Nowadays we know a lot about hormones, we can easily obtain them, some even over the counter, and there’s a lot of marketing. Back when the birth control pill came along, taking hormones was unusual and there would be a whole complex and lengthy explanation to truly explain how the pill worked. So instead they chose to say “the pill tricks you into thinking you’re already pregnant” since it was pretty widespread knowledge that you couldn’t get pregnant if you were already pregnant (not 100% true but 99.99% true). That was enough explanation for most people and as you can see, that explanation has persisted until today even though knowledge about how hormones work and what they do is much more widespread.
So it’s more accurate to say that the pill tricks your body into thinking you’ve already ovulated so you don’t need to ovulate again. That is, the pill contains artificial versions of two main ovarian hormones: estrogen and progesterone. When you ovulate (i.e. the ovaries release a fertile egg), the place where the egg was released forms a corpus luteum cyst that releases progesterone, which is designed for 2 main things: progesterone helps thicken the uterine lining to prepare it for implantation of a fertilized egg, and **it also is a signal to the ovaries that an egg has just been released, which causes all of the other eggs that are currently developing in the uterus to shrivel up and die, an important function because otherwise more eggs might be released, resulting in twins, triplets, quadruplets, etc.**
So basically, if the body detects progesterone, it thinks that an egg has just been released and so it’s time to stop maturing any more eggs, that job is done for the month. So the progesterone in the birth control pill, taken daily, causes any maturing eggs in the ovaries to remain suppressed, never able to fully mature and be released because the progesterone is suppressing them, just as progesterone suppresses egg release naturally each month.
Incidentally, the morning after pill (Plan B) is this type of artificial progesterone, but in a much higher dose than in a birth control pill. This artificial progesterone makes any developing eggs in the ovaries shrivel up and die so you don’t ovulate and therefore can’t get pregnant. If you have already ovulated, then the morning after pill does nothing. And it doesn’t affect that egg so if it gets fertilized, it will implant, Plan B doesn’t cause it to fail to implant, which anti-choice activists call an abortion. But that doesn’t even happen.
Meanwhile, the estrogen in the pill serves multiple functions to prevent pregnancy as well. To understand this, you need to know that a vital part of the whole ovarian cycle is controlled by the pituitary gland, which releases a hormone called follicle stimulating hormone (FSH), which stimulates the ovaries to start having the eggs it contains mature and get ready to be ovulated out. The pituitary releases the FSH, and then as the eggs mature they release estrogen, which is a signal to the pituitary that the FSH message has been received and acted upon. If that estrogen isn’t released by the ovaries, such as in a menopausal woman, the pituitary just continues to pump out more and more FSH until it gets the estrogen signal in return, but if the estrogen is released by those developing follicles, then FSH production decreases in response, which diminishes ovarian stimulation. By including estrogen in the pill, the pituitary gets a fake signal that the ovaries have responded, meaning that the pituitary reduces FSH production. This keeps FSH low enough that the ovaries never start maturing any follicles. Ovarian function thus remains suppressed and eggs don’t mature and aren’t ready for ovulation, and at the same time the progesterone in the pill is also interfering with follicle maturation.
They also have a progesterone-only pill, since progesterone alone can do the job, but the addition of estrogen gives you an extra step to prevent pregnancy. Plus with progesterone suppressing follicles, a woman’s estrogen can become low, which can make her feel not great in some cases.
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