- A cup does not plug you up. Once it’s full, it will leak. Every time.
- The cervix is not all that open. There’s a reason that we get menstrual cramps: because the uterus needs to actively push, and push fairly hard, to expel the menstrual fluids. The blood inside the cup would need to be forced in somehow in order to flow back into the cervix.
- There is no pressure inside the cup. There is even a slight suction (underpressure). It would sooner suck menstrual fluid out of the cervix than force it back in.
- The cup is below the cervix (and in many cases, the cervix is not even in or touching the cup). The fluids would have to travel upwards against gravity to go back into the uterus.
- Backflow is actually quite normal. All women have a bit of backflow. But we don’t all have endometriosis.
- There has never been a single case recorded of endometriosis associated with cup use. I’m not saying it can’t happen; I’m just saying there is no known case.

I hope this is useful.
If you’d like to know more: I’ve pasted a link to an article which cites lots of sources in the website box.

Thanks for the feedback. You make a really good point with not having to be confronted with your period every time you pee. I hadn’t thought about that. Also it’s good to know there are cheaper prices for it out there–I had suspected that might be the case, since it’s just a silicone cup. Maybe I’ll shop around.

I bought my Diva cup about six years ago, shortly after my daughter was born. Without any exaggerating, it has changed my life. Seriously.

And I know that you’re looking at me and thinking “what, seriously? SERIOUSLY?” But it has! I’m pretty much the first person to admit that I hate having my period. I mean, I actively loathe it. The greatest thing about the Diva cup is that instead of thinking about my period every single time I go to pee (which seriously sometimes made me cry), I can think about it twice a day and be done with it.

My cup took me about twenty minutes to get comfortable with, and I’ve used it ever since then. It’s never leaked, save for a few times that I forgot to empty it. Which says a lot right there–I *forgot*.

Because you can wear it for so long (twelve hours is considered normal, but I’ll admit to leaving mine in for more like eighteen to twenty sometimes) you very rarely have to do anything like change it in a public toilet, and even if you do, it’s so fast and easy (remove, dump, wipe, reinsert) that it bugs me far less than tampon or pads, especially since those culminate in having to touch a trash can full of other people’s bodily waste. Ew.

I don’t think that I’m saving tons of money with this, but at the moment, the Diva works out to have cost me about 35c a month. I expect that the cup will be going strong for a few years yet, too, so by the end of it, it’ll probably be even less. I was probably spending $5 a month on tampons before this, which…well, if I’d saved it all, I’d have a few hundred bucks, anyhow.

There was a recent Metafilter thread about the Diva cup–the poster asked for advice, but there’s a lot of discussion in the comments about why the cup is so great, as well as potential downsides.

Also, if you do decide to get one, shop around. Just a quick search led me to LuckyVitamin where it’s $20, and WebVitamins where it’s $19. (No links, as I don’t know anything about the stores and am not shilling.) I bought mine from some motherhood store and got it for $25–lots of legit places have it much cheaper than the official site.