HuffPostWomen Describe The ‘Extreme’ Physical Pain Of Endometriosisshare Women Describe The ‘Extreme’ Physical Pain Of EndometriosisAn estimated 6.5 million people in the United States have endometriosis , a painful condition in which the uterine lining spreads and grows outside of the uterus. But that statistic likely vastly underestimates the prevalence of endometriosis because it’s notoriously difficult to diagnose.