Uterus or Womb, flattened, pear-shaped, hollow organ in the pelvis of the human female and most other mammals. In pregnancy, it is the organ that holds the unborn developing child (see Pregnancy and Childbirth).

The uterus consists of a body, a base or fundus, a neck or cervix, and a mouth. Suspended in the pelvis, it lies with the base directed upwards and forwards and the cervix directed slightly backwards. It is connected to the vagina by the cervix; on either side of it lies an ovary, which produces ova, or eggs, reaching the uterus through the Fallopian tubes. Unimpregnated, the uterus is about 7.6 cm (3 in) in length, 5 cm (2 in) in breadth, and 2.5 cm (1 in) in thickness. In pregnancy the fertilized egg implants itself in the lining of the uterus, where it grows to maturity; its walls are elastic and stretch during pregnancy to hold the developing child. Although the uterus is a muscular organ, it has a lining of soft glandular material which thickens with blood during ovulation, making it ready for receiving a fertilized ovum. If fertilization does not occur, this lining breaks down in menstruation.

The uterus is prone to infection. Endometritis, an inflammation of the mucous lining of the uterus, may affect either the neck or the body of the organ or both. Cancer and fibroid tumours of the uterus are comparatively frequent, as is endometriosis, the occurrence of uterine lining outside the organ.

See also Hysterectomy; Reproductive System.