Thread by @ScienceVet2

Hello again! Friendly neighborhood molecular endocrinologist here. With the news, a couple people have asked me about birth control lately. So, a whirlwind explantion of oral contraceptives for those with uteri.

TLDR Version: "The pill" prevents ovulation

1/

The two critical hormones here are progesterone and estradiol. Unspurprisingly, these are the constituents of oral birth control pills.

2/

Without intervention, estradiol and progesterone drop and start menstruation. This signals the body that there's no fertilized egg. So, you shed the endometrium, the lining of the uterus that a fertilized egg would implant in.

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This, and blood from the small blood vessels surrounding it is what comes out when you bleed.

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While you're bleeding, you get another egg ready, and start producing more estradiol again. When estradiol gets high enough, it triggers another hormone (LH) that causes your mature egg to be released and sent to the uterus.

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At this point progesterone also goes up, and the two together signal the growth of a new thick endometrium, with lots of blood all ready to feed a fertilized egg. Ready for another try at pregnancy!

/6

If there's no fertilized egg by the end of this, we go back to the beginning as estradiol and progesterone drop again.

/7

So what's different if you take BC?

Progesterone and Estradiol remain high all the time.

What does that mean?

If an egg DOES implant, the placenta produces estrogens and progesterone (as well as other things) keeping the levels of these two hormones high during pregnancy.

/8

So, if you artificially keep progesterone and estradiol high, your body behaves as if pregnant.

That means, it doesn't shed the endometrium (no bleeding) and it doesn't release an egg (you can't grow two babies on different clocks in the same uterus for many reasons).

/9

So, the whole idea of "the pill" type birth control pills (combination pills) is to convince your body not to grow a new nest and get an egg set for fertilization.

/10

The "mini-pill" is just progesterone. It causes a thickening of the mucous of the cervix and makes it harder (and more toxic) for sperm to get by to fertilize an egg.

/11

Depending on the person, the mini-pill will sometimes prevent ovulation, but not consistently or reliably without the estradiol to help halt the system.

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So:
"The pill" (conbination pill of estrogen and progesterone) prevents ovulation entirely by convincing your body you're already pregnant.

The "mini-pill" (just progesterone) may prevent ovulation, and makes it hard for sperm to fertilize an egg if it's there

13/

*Neither pill prevents implantation.* If you're on the combination pill and you have sex the sperm won't find anything to fertilize. If you're on the mini-pill and sperm gets by, and an egg has been produced, it can implant and grow (or not, like any fertilized egg).

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For the guys out there with a uterus: Testosterone may (or may not) stop you having periods. This will make it harder, BUT NOT IMPOSSIBLE to get pregnant.

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If your partner has sperm and you don't want to be pregnant, you want to talk to your endo and gyno about how to balance your hormones with some form of birth control. If you DO want to get pregnant, you should also talk to them. Pregnancy involves a lot of hormone surges

16/16

ADDENDUM: IUDs
Hormonal IUDs work by releasing progesterone directly into the uterus in high enough dosages to prevent ovulation

Copper IUDs release copper ions that are toxic to sperm and thickens your mucous.

Neither prevent implantation.

17/16

There are, obviously, a lot more parts and hormones and signal cascades involved in this whole process, but this is the most important part unless you're really interested.

Endometriosis (when the endometrium forms NOT in the uterus) can be treated with BC because, again, it tells your body it's got a fertilized egg so it shouldn't discard the old endometrium (wherever it is).