Endometriosis is a sickness when endometrium cells of the uterus grow outside the uterus. As many as 11% of American women between 15 and 44 years-old may have this sickness. It is most common in women 30-40 years-old. Having endometriosis can make it hard to have a baby. Endometriosis can be treated by a doctor. Treating endometriosis may help a woman get pregnant.