American actress and writer Lena Dunham, best known as the creator and star of the HBO series "Girls," recently announced that she was taking time off from work to rest due to endometriosis, a condition she says is taking a toll on her body. It's a condition that affects hundreds of millions of women and girls around the world, yet most people don't know what endometriosis is or how it can affect a woman's body.

Need a primer? We've got you covered.

What is endometriosis?

Endometriosis is a condition that occurs when the cells that line a woman's uterus, endometrial cells, start to grow outside the uterus. The cells can grow on the ovaries, fallopian tubes, the outer layer of the uterus or the intestines. In rare cases, they can spread to other areas like the lungs, nose, arms or legs.

The big problem with the misplaced growth of these cells is that even though they're not lining the uterus, they continue to act like cells that are lining the uterus. And that means that every month they build, shed and bleed. But unlike the endometrial cells in the uterus, these cells have nowhere to go when they break down, and that can result in a number of painful symptoms.

What are the symptoms?

Depending upon the location of the endometrial cells, endometriosis can cause pelvic pain, painful cramps, pain during sex, backaches, pain during bowel movements, abnormal bleeding and infertility. Endometriosis symptoms may ebb and flow along with a woman's menstrual cycle, increasing as the endometrial cells build up and begin to break down. But some women with the condition feel pain and discomfort at all times.

Getting a diagnosis

Perhaps the most depressing news about endometriosis is that it can be difficult to diagnose. Many doctors dismiss the pain and discomfort felt by women as a normal part of their menstrual cycle, particularly if they only feel it at certain times of the month. This is why studies have found that it takes anywhere from three to 11 years after the onset of endometriosis symptoms for a woman to get a diagnosis.

But the good news is that doctors can use laparoscopy to diagnose endometriosis and develop a treatment plan depending upon the location of the wayward cells. The procedure involves making a small incision in the belly in which a doctor can insert a tiny camera to look for signs of endometrial scarring and buildup.

Is there a cure?

While there's no cure for endometriosis, there are treatment plans that can help minimize the pain and other symptoms associated with the condition. The key here is early diagnosis, because each time the tissue builds up and sheds, it can leave behind scar tissue that can damage other cells in that area. But, as we mentioned above, it can be difficult to get that early diagnosis.

In most cases, women can use over-the-counter pain medications combined with prescription oral contraceptives to control the symptoms of endometriosis. But in some cases, doctors may recommend surgery to remove scar tissue, especially if a woman is trying to conceive.

What is endometriosis?
Here's what you need to know about this common but difficult-to-diagnose condition.