What Really Causes Endometriosis to Spread?
If you've ever been told that endometriosis happens because your period blood flows backward, you're not alone, it's one of the most common explanations out there.
But if that explanation has never quite sat right with you, you're not wrong to question it. Women with endo ask me this all the time, especially those who've noticed their lesions growing over time, or who've heard that a hysterectomy didn't cure someone else's endo, or that men have even been diagnosed with it (yes, men!). If backward period blood were the whole story, none of that would make sense.
So let's actually unpack what researchers currently understand about how endometriosis lesions form, spread, and grow, because understanding this can help you feel more informed and more in control of your own care.
The Theory You've Probably Heard: Retrograde Menstruation
The oldest and most cited explanation is called retrograde menstruation, or Sampson's theory, named after the doctor who proposed it back in 1927. The idea is simple: during your period, some menstrual tissue flows backward through the fallopian tubes into the pelvic cavity instead of leaving the body. That tissue can then implant on nearby structures, like a seed landing in soil, and grow into what we know as endometriosis lesions.
Here's the catch. Research shows that somewhere between 76 and 90 percent of menstruating women experience this same backward flow, yet only a fraction of them go on to develop endometriosis. If retrograde menstruation alone caused the disease, nearly every woman would have it. Since that's clearly not the case, researchers have long known this theory can't be the full explanation, even though it may still play a role for some women.
Why Lesions Persist After Hysterectomy
This is one of the most frustrating and confusing parts of endometriosis for a lot of women. If endo were simply a result of the uterus shedding tissue in the wrong direction, removing the uterus should, in theory, resolve it. But many women continue to experience endometriosis symptoms even after hysterectomy.
This tells researchers something important: once a lesion is established, it can behave somewhat independently of the uterus. Lesions develop their own blood supply, their own local inflammatory environment, and in some cases, distinct genetic changes compared to the original tissue they came from. This is part of why endometriosis is increasingly understood less as leftover tissue and more as a chronic, whole body inflammatory condition, one that involves the immune system, hormones, and genetics, not just the reproductive organs.
What About Endometriosis in Men?
Yes, this really does happen, though it's extremely rare. As of recent literature reviews, only a small number of cases (around 16) have ever been documented. In nearly all of these cases, there was a clear source of estrogen exposure involved, often men receiving high dose estrogen therapy for other medical conditions.
This detail actually supports the bigger picture rather than contradicting it. It suggests that estrogen exposure, immune function, and the local tissue environment matter more than the simple presence (or absence) of a uterus or a menstrual cycle.
So What Actually Determines Whether Endometriosis Develops and Grows?
Instead of one single cause, current research points to a combination of factors that create the right (or wrong) internal environment for lesions to survive and grow:
Immune system function. Normally, your immune system clears out stray cells that end up somewhere they shouldn't be. In endometriosis, this surveillance process doesn't always work as it should, which may allow displaced cells to survive and implant instead of being cleared.
Estrogen exposure. Endometriosis lesions are estrogen-dependent, meaning they respond to and are fed by estrogen much the same way the uterine lining is each cycle. This is a major reason hormone balance is such a central piece of managing symptoms.
New blood vessel growth. For a lesion to grow beyond a microscopic size, it needs to develop its own blood supply. This process, called angiogenesis, is part of what allows small implants to become larger, more established lesions over time.
Genetics and cellular origin. Researchers have also proposed additional theories beyond retrograde menstruation, including the idea that certain cells present since embryonic development can transform into endometrial-like tissue, and that stem cells may play a role in how lesions form in unusual locations, including outside the pelvis entirely.
Inflammation. Chronic, low grade inflammation appears to both result from and further fuel the growth of existing lesions, which is part of why an anti-inflammatory approach to diet, gut health, and lifestyle is so often part of supporting the body through endometriosis.
How Lesion Growth Is Actually Monitored
If you've ever wondered how doctors actually track whether endometriosis is progressing, it comes down to imaging, not symptoms alone. Ultrasound and MRI are the primary tools used to monitor lesion size over time, with MRI being especially useful for deep lesions or when trying to distinguish an endometrioma (an ovarian cyst caused by endo) from other types of cysts. In research and clinical settings, meaningful progression is often defined by specific measurement changes, not just a general sense that something feels different.
This is exactly why, if you've noticed changes in your own symptoms or have had imaging that shows lesion growth, it's so important to bring that directly to your gynecologist or endometriosis specialist. They can compare imaging over time, assess whether your current treatment plan needs adjusting, and make sure nothing is being missed.
The Bigger Picture
Endometriosis isn't a simple plumbing problem, and it was never really about blood flowing the wrong direction. It's a complex condition shaped by your hormones, your immune system, your genetics, and the inflammatory environment inside your body. That's actually empowering information, because it means the work of supporting your hormones, calming inflammation, and supporting your gut and immune health isn't separate from managing endometriosis, it's central to it.
If you're noticing new or worsening symptoms, or changes in lesion size on imaging, please bring that to your medical team. This information is meant to help you understand your body and ask better questions, not to replace the care of your doctor.
Sources & Further Reading
• Halme J, et al. Retrograde menstruation in healthy women and in patients with endometriosis.Obstetrics & Gynecology, 1984. PMC4197400
• Is retrograde menstruation a universal, recurrent, physiological phenomenon? A systematic review. Human Reproduction Open, 2024. Oxford Academic
• Hypersensitive intercellular responses of endometrial stromal cells drive invasion in endometriosis. PMC11729374
• Peritoneal immune microenvironment of endometriosis: Role and therapeutic perspectives.PMC9971222
• Endometriosis in a Man as a Rare Source of Abdominal Pain: A Case Report and Review of the Literature. PMC5833878
• American College of Obstetricians and Gynecologists (ACOG), Endometriosis FAQ
• Cleveland Clinic, Endometriosis: Symptoms, Causes & Treatment
This post is for educational purposes only and is not a substitute for medical advice, diagnosis, or treatment. Please consult your doctor or endometriosis specialist regarding any symptoms or changes specific to your own health.