(CNN) In a Facebook message, Lena Dunham, star and producer of the HBO series "Girls," told fans that she was going to be taking some time off to rest because of her endometriosis, a condition most common in women in their 30s and 40s.

"It certainly can interefere with work life," said Dr. Scott Sullivan, an associate professor at the Medical University of South Carolina. He has not treated Dunham and is not familiar with her case.

How common is endometriosis?

According to the National Institutes of Health , endometriosis affects between 6-10% of women of reproductive age. The condition occurs when tissue that lines the uterus, is found outside of the uterus.

"Most people think that when women have a period, a few of these cells travel through the fallopian tubes into the abdomen and then they end up implanting where they shouldn't be," said Dr. Scott Sullivan, associate professor at the Medical University of South Carolina.

Sullivan said, most of the time those cells just die, but sometimes they implant themselves onto whatever muscle or organ inside. Once implanted, they continue to act like they are still inside the uterus. "They will bleed during a period, like the uterus. Instead of bleeding outside the body, it bleeds inside."

Symptoms

About 11% of women will not experience symptoms. However, for the rest of women with the condition, they typically feel pain during their cycles. For some women, it may develop into more chronic pain. "It can get worse and worse over time. ... It can cause bowel problems, diarrhea, and cramping," said Sullivan.

"Probably no more than 5-10% develop into chronic pain. It's a small group -- but those people really do suffer," said Sullivan.

If not treated early, the cells can develop into growths, causing scars. For some women, it can lead to difficulty in getting pregnant and infertility.

The common way to diagnose it, is to have laparoscopic surgery, a small microscope that is inserted into the belly to look at the abdominal and pelvic organs.

Treatment

Most cases can be managed simply using hormonal birth control, which control the amount of hormone directing those cells. However, that obviously affects the ability to have children.

For those who don't want to use birth control, they can use medicine that actually shuts down your body's hormonal access. "But then they feel like they're menopausal. They feel dry, lose sex drive, " said Sullivan.

Surgery can be used to remove the cells. For some of the most severe cases, hysterectomy may be the only cure, said Sullivan. "Most people progress through stages - conservative, medications, minor surgeries, and then hysterectomy if it gets worse," said Sullivan. Pain may still persist after surgery if not all the cells are removed or if there is scarring from the procedure.

Who develops it?

It's not clear exactly why some women develop the disease and others don't. Women with family members who do have the condition are more likely to develop it. Having a mother, sister, or daughter with endometriosis raises the risk for the condition by about sixfold. However, it is unclear if there is a genetic component at play.

Other factors that may increase risk are starting periods at an early age, monthly cycles that are less than 27 days, or heavy menstrual cycles that last more than seven days.