Merck Manual

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    Endometriosis is a disorder of the female reproductive system. The uterus is composed of 3 layers:

    • The perimetrium (the outer layer)

    • The myometrium (the muscle or middle layer), and

    • The endometrium (the inner layer)

    Each month during a woman’s menstrual cycle, the endometrium grows and thickens to prepare for implantation of a fertilized ovum. When the endometrium reaches its full thickness, an egg is released from the ovaries. If the egg is fertilized, it attaches to the thickened endometrium. However, if the egg is not fertilized, the endometrial layer is sloughed off as a normal part of the menstrual cycle.

    Endometriosis is a condition in which endometrial cells become attached to tissues surrounding the uterus, such as the fallopian tubes, the ovaries, or the large or small intestines.

    During the portion of the menstrual cycle in which the endometrial layer grows, the cells located outside of the uterus also grow. When ovulation occurs and the egg is not fertilized, the endometrial cells slough off both inside and outside of the uterus, causing bleeding, intense cramping, and pain. Endometriosis can also affect a woman’s fertility. A physician can properly diagnosis and manage this disorder.