Everyone knows someone, for whom monthly periods, or menstruation, is an agonizing time. Like Dalila Velez:

"My periods had always been heavy and painful. Painful to the point where there were several occasions when the pain was so excruciating that I would end up at the E.R," the endometriosis patient said.

It wasn't until she was 26 when she had surgery and was then diagnosed with endometriosis, a condition where the lining of the uterus, the endometrium, also grows outside of it, leading to a cycle of bleeding, inflammation, and scarring each month.

Now 34, Velez is having trouble getting pregnant, a common complication.

"You know, going through fertility treatment is quite an emotional journey in and of itself," Velez said. "So, if there was something that I could have done or known about earlier, then perhaps I would have been prepared differently to deal with it or started my journey earlier."

That it took over a decade for Velez to be diagnosed is a major problem, said Northwell Biomedical Scientist Christine Metz.

"Right now, the only absolute way to confirm that an individual has endometriosis is laparoscopic surgery, in which they do an incision in the abdominal cavity and they insert a camera and they take pictures of the inside of the pelvic cavity, and they typically also take out a specimen," said Metz, a biomedical research professor at The Feinstein Institute for Medical Research at Northwell Health.

Metz and her colleagues at the Feinstein Institute for Medical Research have made a discovery that promises to offer women answers about their pain sooner, potentially preserving their fertility.

Blood retrogrades, or travels, out through the cervix and fallopian tubes during menstruation for most women. But for 10 percent, this retrograde blood flow causes endometriosis.

So the researchers analyzed menstrual blood to see why some women have the condition and others do not.

They found that those with endometriosis had significantly fewer immune cells that play an important role in pregnancy, and as a result produced far fewer stem cells necessary to create a healthy womb, a process called decidualization.

"Every month, in order to regenerate a uterine lining, they have to proliferate very rapidly," Metz said. "However, in patients with endometriosis, those cells did not decidualize well."

Metz said that by testing menstrual blood for those cells, they're now able to confirm whether a woman has endometriosis, in just a couple days.

The results of a preliminary study are published in "Molecular Medicine."

Velez took part in the study and hopes more women will join in so future generations will be diagnosed sooner and have earlier access to treatment.

For more information, visit the Feinstein Institute's outreach page on endometriosis.