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If you've ever had one, you know that periods can be uncomfortable, to say the least. While the uterus is busy breaking down and spitting out tissue, the rest of the body is ushering in fatigue, headaches, diarrhea, vomiting, and of course, cramps.

And while cramps are always crummy, some are actually debilitating. They're not stopped by pain medications or hot packs, and they make it hard to get through the day. And, look: Regardless of what you were taught in middle school sex ed, that's not typical.

Sometimes, it can even be a sign of a deeper health condition. One cause of killer cramps is a disease called endometriosis, which happens when tissue that looks like the stuff in the uterus grows other places in the body. Like other areas of the pelvis.

Or… the lungs. Or… the brain. Where it definitely does not belong.

And despite sounding extreme, endometriosis isn't even rare. It affects about 176 million people worldwide. So the more we learn about why it happens, the faster we can start helping.

Endometriosis gets its name from the type of tissue involved in the condition: endometrial tissue. And typically, a layer of it — the endometrium — exists in the uterus and changes as part of a roughly month-long hormone cycle. The layer starts changing when the body releases increased amounts of the hormone estradiol, which is a kind of estrogen.

Then, the layer gets all thick and inflamed, and only stops growing when the body releases more of another hormone, called progesterone. After that, the tissue just sort of hangs out on the off chance a fertilized egg shows up to embed in it. If not, a dip in both estradiol and progesterone causes the endometrium to break down.

And boom, you're on your period! For those with endometriosis, this process can still happen like you'd expect. But in those cases, people also develop patches of endometrium-like tissues outside their uteruses — typically around the pelvis or lower abdomen, but occasionally in more distant places.

These patches are called lesions. And like the true endometrium, they're sensitive to shifting hormones, but not in the same way. Lesions are actually extra sensitive to the growth-causing estradiol, and are resistant to the counteractive progesterone.

So, over time, and with nothing to cancel them out, the lesions become more and more inflamed, and the body can react by covering them in scar tissue. Which makes sense, but isn't great news. Because sometimes, that scar tissue will stick to nearby organs, which can be both incredibly painful and interrupt that organ's ability to do its job.

Thankfully, this might not happen to everyone with endometriosis. There are actually three kinds of endo, each of which affects the body slightly differently. But!

Here's the thing: Endometriosis can only be categorized like that — and treated! — after it's diagnosed. And getting a diagnosis is actually one one the biggest barriers to treatment as a whole. People wait an average of seven years between the first symptoms — which are, remember, debilitating, nagging pain! — and an accurate diagnosis.

A lot of that has to do with stigma around periods and misunderstandings of what's typical. After all, because endometriosis is a hormone-based disease, symptoms usually appear at the beginning of puberty. And if super-painful periods are the only ones you've ever experienced, and if people don't talk very much about their pain, it can be tough to recognize what's too much.

Still, even if somebody has delightful, period-related communication, there's another barrier to diagnosis. It's the fact that, for now, the “gold standard” for confirming that someone has endometriosis is laparoscopic surgery and biopsy. Surgery can be a dramatic step to take, though.

So first, medical professionals usually try every other possible treatment, especially when symptoms aren't life-threatening and the patient is young. The problem is, doctors sometimes treat symptoms instead of the root cause. Like, without a proper diagnosis, they might just try and treat bad cramps instead of actual endometriosis.

So, their treatments might not work. And a patient might spend years going from option to option, until finally, diagnostic surgery is suggested and the doctor confirms that, hey, yep, that's endo. After that, treatments can vary.

In some cases, pain can be managed with painkillers and hormonal birth control. And in others, doctors recommend things like surgery to remove lesions, artificially-induced menopause, or removing the uterus and ovaries entirely in a hysterectomy. Unfortunately, though, none of these have proven 100% reliable.

Even after a hysterectomy, some people continue to have lesions and pain. As for why? Well, there are a lot of aspects of endometriosis researchers still don't fully understand including how lesions form in the first place.

There are some ideas, though. Like, one involves something called retrograde menstruation, which sounds weird, but it's actually really common. It's where menstrual tissue and blood just happens to flow backwards through the fallopian tubes instead of out of the vagina.

Since the fallopian tubes aren't directly connected to the ovaries, that fluid can escape into the pelvic area. And while this doesn't always cause problems, one hypothesis says that sometimes, chunks of endometrium can implant in the organs they run into and form lesions. One lab actually studies endometriosis in baboons by artificially inducing retrograde menstruation, so there is some evidence for this, although nothing has been confirmed in humans.

The other leading hypothesis is that adult stem cells (which have the potential to turn into a few different kinds of cells) might turn into endometrial cells in the wrong place. Which would explain why lesions have been found all the way up in people's brains and lungs. Additionally, there might be a genetic component to endometriosis, too, since people whose family members have the condition are more likely to have it themselves.

The good news is, research is ongoing. And with every new paper, we'll likely learn more about where these lesions come from and how to effectively treat them. So someday, maybe those killer cramps could be a thing of the past — at least, when it comes to endometriosis.

Unfortunately, severe cramps can still show up thanks to a whole host of other conditions, including adenomyosis, PCOS, and fibroids. So if any of this rings a bell for you, talk to your doctor. Because even though periods can be inconvenient, they don't need to interfere with everyday life.

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