The most widely accepted hypothesis is that endometrial cells are transported from the uterine cavity and subsequently become implanted at ectopic sites. Retrograde flow of menstrual tissue through the fallopian tubes could transport endometrial cells intra-abdominally; the lymphatic or circulatory system could transport endometrial cells to distant sites (eg, the pleural cavity). Another hypothesis is coelomic metaplasia: Coelomic epithelium is transformed into endometrium-like glands.

Microscopically, endometriotic implants consist of glands and stroma identical to intrauterine endometrium. These tissues contain estrogen and progesterone receptors and thus usually grow, differentiate, and bleed in response to changes in hormone levels during the menstrual cycle.

Incidence of endometriosis is increased in 1st-degree relatives of women with endometriosis, suggesting that heredity is a factor. Incidence is also increased in women who delay childbearing, who have shortened menstrual cycles (

< 27 days) with menses that are abnormally long (>

8 days), or who have mullerian duct anomalies.

Reported incidence varies but is probably about 10 to 15% in actively menstruating women aged 25 to 44. Average age at diagnosis is 27, but endometriosis also occurs among adolescents. About 25 to 50% of infertile women have endometriosis. In patients with severe endometriosis and distorted pelvic anatomy, incidence of infertility is high because mechanisms of ovum pickup and tubal transport are impaired. Some patients with minimal endometriosis and normal pelvic anatomy are also infertile. In these patients, fertility may be decreased because incidence of luteal phase dysfunction or luteinized unruptured ovarian follicle syndrome (trapped oocyte) is increased, peritoneal prostaglandin production or peritoneal macrophage activity is increased (resulting in phagocytosis), or endometrium is nonreceptive.

Potential protective factors seem to be multiple pregnancies, use of low-dose oral contraceptives (continuous or cyclic), regular exercise (especially if begun before age 15, if done for > 7 h/wk, or both).