Senie Byrne began menstruating at 15, and she quickly became an expert in pain management. Every month, it felt as if 1,000 darts had plunged into her abdomen; she sometimes passed out from the cramps. She was often nauseated, constipated and exhausted. At times, she had diarrhea. She said she saw 22 doctors over the years, but none took her seriously.

“They told me that it was all part of being a woman, and that there was nothing they could do for me,” said Ms. Byrne, 29, who works in financial services in Leesburg, Va. One doctor gave her a prescription for Prozac, telling her it was all in her head.

Finally, when she was 21, a doctor performed a laparoscopy to see what was going on. The diagnosis: severe endometriosis, a hormone and immune system disease in which uterine tissue is found outside the uterus, causing lesions in the ovaries, fallopian tubes, abdominal lining, bowel or bladder. While it is not known how or why uterine cells form in other parts of the body, the lesions can cause inflammation and debilitating pain.

Ms. Byrne said she was somewhat relieved to finally have a name for what had been ailing her. “I knew something was wrong when I was 15, but no one listened to me,” she said.

An estimated 89 million women worldwide, including 6.5 million in the United States and Canada, have endometriosis, according to the Endometriosis Association, a research and advocacy group. Symptoms can include debilitating periods, painful bowel movements with menstruation, inflammation, internal bleeding, scar tissue and infertility.

But those numbers are rough, experts say, because many young girls with endo, as it is called, do not find out until years later. In fact, research from the Endometriosis Association estimates that it can take up to 10 years from the onset of pain for a doctor to give a diagnosis of endometriosis, largely because many physicians do not believe the disease affects teenagers. By that time, the damage has been done.

“The medical teaching on endometriosis was that it’s a disease of women in their reproductive years, not adolescents,” said Mary Lou Ballweg, the president and executive director of the Endometriosis Association. “Many times, we hear that girls are told they’re too young to have the disease, they’re trying to get out of school, or that they’re exaggerating. Add the misconception that pain with menstruation is normal, and you get a bundle of confusion. And not the least, most gynecologists are uncomfortable treating adolescent gynecological problems, and pediatricians don’t.”

Jill Fuersich, 31, a preschool teacher in Manhattan, first had her period when she was 12. She often missed school days and social events because she was in agony. “The doctors said that it was normal to have a period that was painful,” she said.

She bounced among gynecologists and gastroenterologists for a decade until she finally received a endometriosis diagnosis at 23. She was given birth control pills and a progesterone IUD, which were supposed to ease the symptoms. They did not.

She has since had four operations, the most recent a four-hour excision procedure to remove adhesions and fibroids. “I feel O.K.,” said Ms. Fuersich, a co-founder of a support group, Endo Warriors. “It was very discouraging that I had to be in pain for so many years before I got any real help.”

Many doctors are not educated about the disease in adolescents. “Why is the gap so huge between scientific understanding of the disease and the treatment people are getting?” said Dr. Grace Janik, a reproductive surgeon in Milwaukee. “There’s a lack of understanding in the medical community of what options are available and what symptoms to look for.”

Endometriosis has no known cure, although surgery to remove endometrial lesions may help temporarily. If endometriosis occurs in the bowel, a bowel resection is another option, although insurance often does not cover it.

Notably, being pregnant sometimes offers relief, but many women with endometriosis are infertile. A hysterectomy, surgical removal of the uterus, also may help, but not always.

“A lot of women who are looking for relief from endo will undergo a hysterectomy, and that won’t necessarily provide a relief from their symptoms,” said Dr. Linda M. Nicoll, an assistant professor in obstetrics and gynecology at NYU Langone Medical Center.

A practice committee of the American Society for Reproductive Medicine has issued guidelines for the treatment of pain associated with endometriosis, but the document makes clear that no studies have directly compared medical therapies and surgery, so one is not recommended over another.

Those involved in fighting the condition say there is not a one-size-fits-all solution.

“The disease is highly individual,” Ms. Ballweg said.

Bern Bingham, 22, a nurse in Massapequa, N.Y., had debilitating monthly pain since she was about 12. But she was 20 by the time she received a formal endometriosis diagnosis. The lesions cover her liver, chest wall and ovaries. A doctor prescribed Lupron, a drug that suppresses hormones, but she did not want to deal with the side effects. “I didn’t want to be in menopause at 20,” said Ms. Bingham, the founder of Knock Out Endo, a support group.

Over the last three years, she has had three operations, including a bowel resection. But she still suffers during ovulation and menstruation.

After two outpatient and two inpatient operations, Ms. Byrne also still had pain. She mulled becoming pregnant to alleviate the symptoms, but a fertility specialist determined her eggs were unviable. At 22, she decided to have a hysterectomy.

For five and a half years after that, Ms. Byrne had little pain. But recently, she has started experiencing symptoms as if she were having her period.

Ms. Byrne, who runs a Facebook group called Childless Support, said that she sometimes became enraged by her situation.

“If the doctors had taken me seriously at 15,” she said, “who knows would have happened to me at 22?”

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By Abby Ellin