Sometimes referred to as an "upside down pear", the uterus is a muscular organ located in the pelvis between the bladder and the rectum.

It has an internal cavity known as the endometrial cavity that is continuous with the cervical canal, the vagina below and the two openings in the upper portion leading to the fallopian tubes, right and left. The lining of the cavity is called the endometrium.

During the menstrual cycle, which is regulated by the action of ovarian-secreted hormones, estrogen and progesterone, the endometrial lining thickens and the uterus sheds both the unfertilized egg and the thickened endometrium. The uterus varies in size depending on the woman's age and whether she has had previous pregnancies. The average adult uterus is about 3 inches long, 2 inches wide, and 1 inch thick. After menopause, the uterus shrinks to the pre-adolescent size.

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