How does a mirena work?

January 9, 2023 4:49 AM

I have a mirena, I have questions.

So I'm on my second Mirena. First one to deal with heavy periods before I was sexually active, got married, removed, two kids, back again. It seems to work well.

It's just, I have a niggling doubt as to how it works. Pregnancy has made me more pro choice than I ever was, but I still have a strong faith background and would prefer not to have accidentally aborted. I hadn't really researched it as contraception (having first had it for other reasons) and had a moment of soul searching panic on our honeymoon that we resolved, but I'm back there again.

This is surprisingly hard to google without turning up gross anti-choice resources that guilt trip me. I need to get this sorted in my head though as I assume ovulation is when my libido picks up and it's a bit of a downer on my marriage to be worrying about "maybe I'm ovulating so we should skip it" especially after long new mirena spotting.

How I understand how it works:
It makes it hard for the sperm to get through, it may suppress ovulation, it thins out the uterine lining (yay lighter periods, of course.) How likely is it that the mirena causes, for lack of a better term, an abortion/miscarriage?

posted by freethefeet to Health & Fitness (9 answers total)

I think there aren’t statistics that you’re going to be able to rely on here. That is, the primary mechanism of action is definitely preventing fertilization — the Wikipedia page links to resources that explain how it works, but you know that already, it’s in the post. Here’s a study saying that when researchers checked the fallopian tubes of 115 women who were having sex unprotected, they found fertilized (pre-implantation) eggs in about half of them, and when they checked 56 IUD users who were having sex they found no fertilized eggs: here. So that’d give you a sense of the odds of fertilization — low enough that it didn’t happen once in a sample of 56.

But can you be absolutely sure: (1) that if you have sex with an IUD an egg will never be fertilized, and (2) that if it is, its odds of implantation will be just as high as if you didn’t have the IUD? I don’t think there’s any way to get to that level of certainty. If it bothers you, you could use condoms when you believe you’re in your fertile period — that still doesn’t get you to certainty, because condoms can break or leak, but at some point piling improbability on improbability has to be consoling.
posted by LizardBreath at 5:32 AM on January 9, 2023 [2 favorites]

How likely is it that the mirena causes, for lack of a better term, an abortion/miscarriage?

I don't know what better term you're trying to find, but this isn't how it works. Not at all.

The Mirena specifically, and hormonal IUDs generally, are more effective at preventing pregnancy than tubal ligation. More effective than getting your tubes tied.

It stops you from becoming pregnant at all--you listed the ways it does this. To poorly quote Raising Arizona, it makes your womb a hostile place where seed can find no purchase. If you never get pregnant, there's nothing to abort.

There is a very small chance that a fertilization can still occur--sperm makes it past all the barriers that are in place, you ovulated anyway, and everything lined up just so. Because, again, the uterus is not a friendly place for a blastocyst, if this cell does implant, it could be somewhere extremely dangerous for you like your fallopian tube, which could kill you.

Using a Mirena, you either 99.9% have no risk of any embryo forming at all, or you have an extremely tiny very small chance of requiring a (most likely medical) abortion to save your life. Which I certainly hope your faith makes an allowance for.
posted by phunniemee at 5:39 AM on January 9, 2023 [9 favorites]

Best answer: And you clearly don't sound like a person who believes this, but I just want to be very clear that women who have IUDs aren't walking around running abortion factories all day in their uterus, as some Republican congressman would have you believe. That analogy is about as close to real as the idea I'm running a pizza restaurant out of my house simply because I've got flour and tomato sauce and an oven in my kitchen. It's missing several steps, is factually untrue, and it sounds absurd to anyone who knows what either a pizza or a restaurant is.
posted by phunniemee at 5:47 AM on January 9, 2023 [19 favorites]

Best answer: I work in sexual and reproductive public health. I think it may help to clarify your own values first (through googling you may find “VCAT” or values clarification exercise, usually done in a group setting but may still help you walk through some ideas). Once you have some further clarification on your own beliefs, then choose a birth control method (if any) that fits in with your beliefs rather ing than shoe horning your beliefs around a specific type of birth control.

Phunniemee’s info is correct in how Mirena works, largely to prevent ovulation, prevent fertilization, but if both occur, to prevent implantation in the uterine wall.

If preventing a blastocyst from implanting is never acceptable for you, then you may want to consider another method(s) of birth control than a hormonal IUD.
posted by raccoon409 at 5:52 AM on January 9, 2023 [1 favorite]

Best answer: Another way to think about this — imagine you were using a barrier method of birth control, with a failure rate because everything has a failure rate. Either it prevents fertilization—no moral problem given your beliefs—or it doesn’t work at all. Then say you read that drinking coffee lowers the odds that a fertilized egg will be implanted. Would that not bother you, because you’re not trying to get pregnant anyway, and the strong odds are that there aren’t going to be any fertilized eggs to be affected by that mechanism? Or would you give up caffeine because any decrease in the odds of implantation as a result of something you’re doing voluntarily is a moral problem for you?

If you wouldn’t give up caffeine, I think there’s a good argument that the possible effect of an IUD on implantation in the unlikely circumstance that its primary mode of operation fails shouldn’t bother you.
posted by LizardBreath at 6:15 AM on January 9, 2023 [14 favorites]

The manufacturer says: "To prevent pregnancy, Mirena: Thickens mucus in the cervix to stop sperm from reaching or fertilizing an egg. Thins the lining of the uterus and partially suppresses ovulation."

These things prevent pregnancy, it is not causing a miscarriage.
posted by tiny frying pan at 6:18 AM on January 9, 2023 [2 favorites]

Response by poster: Thanks everyone, I knew metafilter would be the place to get compassionate and factual advice.

I didn't feel bad about any potential blastocysts that didn't implant when we were attempting to conceive our kids, the coffee thing was helpful.
posted by freethefeet at 2:29 PM on January 9, 2023 [3 favorites]

Best answer: If you want a deep dive, Reveal did an excellent hour of reporting last year on how "birth control = abortion" is being pushed by right wing zealots, why that's not the reality of things, how "preventing implantation" language came to be added to Plan B packaging (surprise! anti-abortion activists who managed to get appointed as science advisors!), and a lot more. The episode stuck with me because of how I felt more infuriated than normal after a Reveal episode:

https://revealnews.org/podcast/the-long-campaign-to-turn-birth-control-into-the-new-abortion/
posted by slagheap at 9:27 PM on January 10, 2023

Response by poster: slagheap - yep, I'm a person of faith, which means that sometimes I'm around some gross right wing ideas - the weaponising of the Christian community on abortion/birth control is also something to look at, too- this article at politico is interesting reading.
posted by freethefeet at 11:18 PM on January 13, 2023