Pregnancy and endometriosis exhibit a curiously antagonistic relationship. Women who give birth in their 20s are much less likely to suffer from this condition than women who conceive in their 30s or those who never have children. Endometriosis is defined by the presence of endometrial cells outside of their normal location within the uterine cavity. Reflux of menstrual blood into the abdominal cavity and subsequent implantation of the endometrial cells in the abdominal cavity is the most widely accepted and intuitively obvious theory for the origin of endometriosis.
Endometriosis causes either pelvic pain or infertility or both. While a benign condition, the extent of pelvic involvement by endometriosis is staged as if it were a malignant disease. The most common sites for endometriosis include the ovaries where it can cause formation of cysts known as endometriomas.
Since women do not menstruate while pregnant and breastfeeding, there is no re-seeding of endometriosis implants during that time. In addition, the hormonal milieu of pregnancy, with very high levels of progesterone, further suppresses pre-existing disease. Thus endometriosis and its associated symptoms of pelvic pain and infertility are indeed both prevented and alleviated by pregnancy.