当子宫内膜腺体和间质出现在子宫以外时，称为子宫内膜移位症。近10%的女性可能都有这种情况。即使有少数病灶也可导致痛经和月经不调。左图显示了几个常见的子宫内膜移位的部位。发生在卵巢反复出血，卵巢体积增大，形成囊腔，内含粘稠的咖啡色液体，称之为巧克力囊肿。

子宫内膜移位症：发生部位（模式图） | 镜下 | 移位于结肠（镜下） | 移位于子宫后浆膜下(大体) | 移位于子宫后浆膜下（肉眼放大） | 卵巢切面示巧克力囊肿

When endometrial glands and stroma are found outside the uterus, the condition is known as endometriosis. Up to 10% of women may have this condition. It can be very disabling and painful, even when just a few foci are present. Diagrammed here are typical locations for foci of endometriosis. Sometimes the old dark brown blood collects over time from repeated hemorrhage in a cystic space in the ovary and produces a so-called "chocolate cyst".