Lena Dunham has endometriosis. Here are 5 facts about the disorder.

Lena Dunham just announced that she's taking time off from a Girls promotional tour because of a medical condition called endometriosis.

"As many of you know I have endometriosis, a chronic condition that affects approximately 1 in 10 women’s reproductive health," she wrote on Facebook this week. "I am currently going through a rough patch with the illness and my body (along with my amazing doctors) let me know, in no uncertain terms, that it’s time to rest."

In a recent edition of her Lenny newsletter, she also explained that her endometriosis went undetected for years while she struggled with debilitating pain.

This isn't surprising at all, considering that endometriosis is mostly mysterious and difficult to diagnose — yet, as Dunham notes, incredibly common in women of reproductive age. Here are five important facts about the condition.

1) Endometriosis is really hard to diagnose

Endometriosis basically means that the tissues of the uterus are growing outside the uterus. Every month, a woman's uterine lining sheds during her period if she's not pregnant. With endometriosis, the displaced tissue "continues to act as it normally would — it thickens, breaks down and bleeds with each menstrual cycle," according to the Mayo Clinic.

In some women, this tissue growth happens without them even noticing. In others, it's a painful, chronic condition. Because the extra tissue has no way to exit the body — regular uterine lining would shed through the vagina during a period — it builds up and becomes trapped, leading to pain and, in some cases, infertility.

In order to definitively diagnose the condition, doctors need to peer inside the pelvis through a laparoscopy. (Imaging is rarely helpful in getting a clear diagnosis.) This involves making a small incision in the belly and inserting a tube with a tiny camera to see if the pelvic organs show signs of tissue scarring and buildup.

Since laparoscopy is an invasive surgery, many women go undiagnosed for years. It often takes between three and 11 years after the onset of symptoms to diagnose the disorder.

2) The extra tissue doesn't only affect the pelvis — it can grow on the nose and legs too

Most of the time, this extra tissue grows in the pelvis. But researchers have found that in rare cases, it can grow in remote sites on the body. There have been cases where endometrial tissue has been found in the nose, the lungs, and even on the arms and thighs.

As one sufferer describes, "The same type of cells that grow in my womb grow in other parts of my body, on my ligaments, my bowel, my liver, and when I bleed, they bleed into places they shouldn’t and the blood can’t escape. I’m riddled with scar tissue, inside and out. My organs are stuck together with scars called adhesions. I’ve had seven surgeries over the past eleven years and there will be more. I’ll never be bikini ready."

3) There's still no cure for endometriosis

According to the Practice Committee of the American Society for Reproductive Medicine, "Endometriosis should be viewed as a chronic disease that requires a life-long management plan with the goal of maximizing the use of medical treatment and avoiding repeated surgical procedures."

The three main ways endometriosis manifests are through pelvic pain, infertility, and pelvic masses such as cysts, scar tissue, and fibroids. Again, some women have no symptoms all, but most seem to experience pain in the lower belly.

According to the American Congress of Obstetricians and Gynecologists, some key signs of endometriosis include:

• Pain before and during periods
• Pain during sex
• Infertility
• Fatigue
• Painful urination during periods
• Painful bowel movements during periods

There's no cure for the condition, so therapies — painkillers, surgery, hormonal medical therapies, oral contraceptives — are used to abate symptoms and improve fertility.

For mild pain, some women use nonsteroidal anti-inflammatory drugs and estrogen-progestin oral contraceptives. The latter seem to minimize symptoms. If those don't help, some women go on to use stronger hormonal therapies that stop monthly periods, or synthetic steroids.

In more severe cases, a doctor may suggest surgery to remove extra tissue — particularly if a woman is trying but unable to get pregnant. (It's important to note, though, that many women with endometriosis go on to have healthy pregnancies.)

4) Doctors aren't sure why some women get it but not others

No one knows exactly what causes endometriosis, but doctors have some theories:

Retrograde menstrual flow: During a period, when a woman is shedding her uterine lining, some of that tissue can flow through her fallopian tubes and into her pelvis, which researchers think may cause endometriosis. But doctors have also found that the rate of retrograde menstruation is about the same in women with and without endometriosis, so they think additional factors spur on the disorder in some.

Genes: The disorder runs in families, so doctors believe there's a genetic component. If a woman's mom or sister had endometriosis, she's believed to be at a higher risk.

Immune system dysfunction: Problems with the immune system can hamper a woman's ability to clear her menstrual debris, leading to endometriosis.

Environmental factors: There's some evidence to suggest environmental exposures to certain chemicals may contribute to one's risk of developing endometriosis.

5) Pregnancy seems to temporarily fix the problem

Adding to the mystery, for reasons the scientific community has yet to understand, pregnancy seems to clear women of endometriosis symptoms. But after giving birth, the symptoms come back, typically following breastfeeding.

Overall, for about a quarter of women who are diagnosed with endometriosis, the symptoms just clear up on their own.

For women with more severe cases, endometriosis can represent lifelong suffering, involving chronic pain and even many surgeries that don't seem to help.

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