The Flex guide to endometriosis & how it affects your period


Photo by Georgie Wileman via THISISENDO. Follow Georgie’s global awareness campaign on Instagram @thisisendoglobal.

“Living with endo is best explained by those who know it best: the patients themselves.”

Understanding the hidden pain of endometriosis

TL/DR: Endometriosis affects nearly 200 million people worldwide, and it can take almost a decade to be diagnosed. Untreated, endo leads to period pain, irregular periods, and in severe cases, even infertility. Endo both affects your period and is affected by your period — and here’s why.

Last month, Olympic skater extraordinaire Tara Lipinski made waves when she posted on Instagram about her struggle with endometriosis, inviting fans into her hospital room with a series of powerful photos depicting her medical journey with the disease. She opened up about the intermittent pain that she felt for years, and how it wasn’t until recently that she sought out treatment for her symptoms. 

Tara wasn’t alone in waiting for a diagnosis. The average delay to diagnosis – the time between noticing symptoms and being told by a medical provider that what you’re feeling is, in fact, due to endometriosis – is 6.7 years.1 For some women, that can mean nearly a decade of being shuttled from one provider to another, all the while experiencing at-times debilitating symptoms. 

Endometriosis affects around 1 in 10 individuals assigned female at birth and of reproductive age, yet it is sorely underfunded in research dollars and not talked about nearly enough.2 Here’s the long and the short about endo and how it affects your period.

What is endometriosis? 

To understand what’s happening in endometriosis, first try to visualize the female reproductive system in all its glory. The vaginal canal leads up to the cervix, which is the gateway to the uterus, a T-shaped structure that serves as the main center for menstruation and reproduction. The lining of the uterus is called the endometrium – and it plays a key role during the menstrual cycle.3

During a typical menstrual cycle, the nutrient-rich endometrium proliferates, or builds up, over the first couple of weeks of your cycle. Based on signaling from FSH (follicle stimulating hormone) and estrogen, the endometrium thickens (or becomes vascularized) with nutrient-rich blood. Basically, it’s preparing itself to be a warm, wet, comfy home in the event of a pregnancy. 

Then, ovulation happens – ta da! A brand new egg is released by the ovaries. If that egg is fertilized by sperm, the endometrium will continue down the building pathway. If the egg remains unfertilized, the endometrium will instead enter what is known as the secretory phase. During this time (the ~2 weeks before your period), the endometrium stops thickening and instead begins to secrete stored glucose.

It’s during the menstrual phase of your cycle that the endometrium really takes center stage. Hormonal signaling triggers those familiar uterine cramps that allow your body to shed the endometrium. That’s what brings about the bleeding that we all recognize as our favorite time of the month. 

So: Endometriosis obviously has something to do with the endometrium – but what, exactly, is it? 

Endometriosis is a condition in which endometrial-like tissue (containing cells similar to those that make up the endometrium) begins to grow outside of the uterus, which isn’t expected normally. In most cases, this out-of-place tissue is found somewhere in the lower abdomen, or pelvis, but it can technically appear anywhere in the body. A few different hypotheses exist to explain why this might happen:4

What causes endometriosis?

Meyer’s Theory suggests that endometrial cells become mutated, either spontaneously or through a genetic condition. This would allow these cells to grow without limits, resulting in the growths or polyps often seen with endometriosis. This explanation makes sense in theory, but it doesn’t explain how the endometrial cells move to where they’re found.

Sampson’s Theory suggests that endometriosis is due to retrograde menstruation, which refers to a “backflow” of menstrual blood during your period (not quite Mercury retrograde, but still not a positive thing). Basically, it’s thought that some of the endometrial cells shed during your period might travel out through the fallopian tubes and into the abdomen – moving in the opposite direction that they’re supposed to (i.e. out of your body). Again, this theory sounds compelling in terms of its explanation as to how these cells end up outside of the uterus. However, given the fact that 70-90% of menstruators experience retrograde menstruation and only 3-10% of menstruators are diagnosed with endometriosis, it’s not entirely plausible.5 

Halban’s Theory provides an alternative explanation for cell spread, arguing that the endometrial cells move to distant parts of the body through lymphatics, which are the tunnels that exist all over your body through which your immune cells travel. This could potentially make sense for the strange, distant sites of endometriosis polyps, but doesn’t explain why endometrial cells would also end up local to the uterus.

If there’s anything to be learned from these theories, it’s that, at this point, we really don’t know what causes endometriosis. It definitely warrants further research (hello, Congress!).

How does endometriosis affect the menstrual cycle? 

Like we’ve talked about, the endometrium is highly responsive to hormones, which makes sense for its role in menstruation and reproduction.2 But when endometrial-like tissue spreads outside of the uterus, it can result in painful symptoms that may last for years without treatment.

The endometrium is like your ovaries’ sidekick. When the ovaries say “Estrogen!” the endometrium says, “How thick?” When the ovaries say “Progesterone!” the endometrium is already scrambling to let out that stored glucose. But when endometrial cells are scattered outside of the uterus, like in the fallopian tubes, the ovaries, or even in far off locations like the omentum (a fatty appendage of your stomach), it can cause serious problems. It is essentially hormonal overload, in some cases leading to bleeding, scarring, and bands of scar tissue called adhesions. 

This whole process is responsible for the symptoms that many endo patients experience, one of which involves changes to the menstrual cycle. Patients with endometriosis often experience heavy menstrual bleeding and/or irregular periods.6 Periods may also last longer since there is more tissue to slough off; some individuals even experience bleeding in between periods. 

If you have an irregular period – whether it’s here one month and gone the next, or your cycle keeps changing in length – it’s always a good idea to speak with your healthcare provider, even if you don’t have any other symptoms. 

Endometriosis symptoms: Period pain & irregular cycles

For patients with endometriosis, pain is the common thread. Many individuals diagnosed with endo experience pelvic pain, period pain, and pain during sex.6 However, it’s important to remember that each person’s experience with endometriosis will be highly individualized: Endo pain can vary significantly from one person to another in terms of its severity, location, time of the month when it’s most noticeable, and how it changes over time. 

In certain severe cases, endometriosis can also lead to infertility. If you experience any of the below symptoms, consider reaching out to your healthcare provider for support.

  • Heavy menstrual bleeding (i.e. bleeding that forces you to change your preferred form of period protection every two hours or less, bleeding that lasts longer than 7 days, or bleeding that involves clots larger than the size of a quarter) 7
  • Painful periods, including uterine cramps and cramps outside of the pelvis (e.g. lower back, stomach, upper legs, etc)
  • Pain with sex
  • Pelvic pain
  • Nausea, vomiting, fatigue, diarrhea, bloating, etc., either on or around your menstrual period

Living with endo: Tips & strategies

There is currently no cure for endometriosis. It’s no wonder, therefore, that an endometriosis diagnosis can be devastating. Your doctor may offer you a list of potential treatments geared towards reducing pain, but no treatment is guaranteed to be 100% effective (and many entail long-term side effects). We recommend doing plenty of research before committing to a plan. 

The good thing is, if you’ve joined the ranks of endo warriors around the globe, you’re not alone. 

Living with endo is best explained by those who know it best: the patients themselves. Padma Lakshmi, the chef, model, and activist, co-founded The Endometriosis Foundation of America alongside her gynecologic surgeon in 2009, which aims to be a resource for patients seeking support. There are also a number of online and (pre-COVID) in-person support groups, some of which are rooted in traditional, allopathic medicine, and others which are more closely tied to integrative or holistic wellness.  

Choosing menstrual products that work for you, especially ones that can accommodate heavy bleeding, may help with symptom management. Self-care practices may also help with cramps (along with a healthy dose of Tylenol or Aleve if your doctor gives you the go-ahead). Endo pain is no joke – if you’re suffering, don’t go through it in silence. Try to fill in your close friends, family members, teachers, and employers about your condition so you can ask for downtime when you need it. 

Whichever way you seek support through your endo journey, know that there is a community out there for you. Don’t be afraid to speak up for yourself and your pain: You should always feel empowered to seek the information, resources, and support that you deserve.

This article is informational only and is not offered as medical advice, nor does it substitute for a consultation with your physician. If you have any gynecological/medical concerns or conditions, please consult your physician. 

© 2021 The Flex Company. All Rights Reserved.

References (Click to open/close)

  1. Nnoaham, K. E. et al. (2011, June 30). Impact of endometriosis on quality of life and work productivity: a multicenter study across ten countries. National Center for Biotechnology Information, U.S. National Library of Medicine. Retrieved from
  2. American College of Obstetricians and Gynecologists. (2019, Jan). Endometriosis. ACOG. Retrieved from College of Obstetricians and Gynecologists. (2019, Jan). Endometriosis. ACOG. Retrieved from
  3. Monis, C. N. (2020, Sept 15). Menstrual Cycle Proliferative And Follicular Phase. National Center for Biotechnology Information, U.S. National Library of Medicine. Retrieved from
  4. Carpinello, O.J. et al. (2017, Oct 27). Endotext [Internet]. National Center for Biotechnology Information, U.S. National Library of Medicine. Retrieved from
  5. Endometriosis: What is Endometriosis? Endometriosis Symptoms, Treatment, Diagnosis. (n.d.). UCLA Health. Retrieved from
  6. Mayo Clinic Staff. (2019, Oct 16). Endometriosis: Symptoms and Causes. Mayo Clinic. Retrieved from Clinic Staff. (2019, Oct 16). Endometriosis: Symptoms and Causes. Mayo Clinic. Retrieved from
  7. Heavy Menstrual Bleeding. (2016, June 1). American College of Obstetricians and Gynecologists. Retrieved from