The uterus is a hollow, thick-walled, pear-shaped muscular organ in the female reproductive system. During pregnancy the uterus expands to accommodate a developing embryo. It is located between the urinary bladder in front and the rectum behind, and sits above the vagina.
The lower narrow portion of the uterus is called the cervix and it protrudes downward into the opening of the vaginal canal. The vaginal canal extends downward to the external female genitalia.
The uterine tubes, or Fallopian tubes, extend from either side of the uterus and act as a channel for eggs from the ovary to travel to the uterus.
When an egg is fertilized (joined with sperm), it becomes embedded in the wall of the uterus (whose lining becomes thickened) where the fertilized egg grows into an embryo and later a fetus.
If an egg is not fertilized, the thickened uterine lining sloughs off in a process known as menstruation.