Endometriosis is when tissue that is similar to the normal lining, inside the uterus that gets shed during a period, grows outside of the uterus. This tissue is similar to the normal lining, but is not the same. Research shows this and there is a lot of misinformation about this everywhere on the internet and in support groups. It's important to know that it's not the same. However, because it's so similar to the normal lining, it also reacts the same to the hormones during a period. This can create the same bleeding and inflammation that a period brings, but there is no escape for the blood to go. So, it stays in the abdomen or wherever this tissue is located. There is no known cause of endometriosis. However, research shows that it is present at birth and is formed during pregnancy.