The uterus or womb is the hollow muscular organ in women in which a baby grows. It is located in the pelvis and is loosely tethered to the pelvic walls by two ligaments on each side, the round and broad ligaments, giving it a high degree of mobility. It leans forwards when the rectum is full and backwards when the bladder is full. During pregnancy it expands upwards as far as the ribs. In a non-pregnant woman the uterus looks something like an upside-down pear. It is about 7.5 cm. long and 5 cm. wide. The cavity of a non-pregnant uterus is small and narrow, virtually a slit.
The upper part of the uterus is called the body, and is attached to the two egg-conducting Fallopian tubes. It narrows at the lower end to form the cervix, or neck, which protrudes into the vagina and provides a passage for sperm to enter and menstrual blood to flow out.
The wall of the uterus is made up of three separate layers. The outer layer is a tough protective covering called the perimetrium. In the middle is a thick layer of muscle called the myometrium, while the inner lining consists of a blood enriched mucous membrane called the endometrium.
Each month the endometrium thickens to prepare for the implantation of a fertilised egg. If this does not eventuate, all but the deepest part of the endometrium is discarded, leading to the monthly menstrual period. This takes place about 14 days after an egg has been released. The menstrual flow consists of the liquefied dead endometrium together with some blood lost in the process. If fertilisation, or pregnancy, does occur, the embryo is implanted in the endometrium and nourished by the mother’s blood supply. The mother’s and the embryo’s blood circulations interact through the placenta.
The muscles in the myometrium are among the strongest in the human body. They expand to accommodate the growing foetus, and when the time comes for the baby to be born they engage in a series of contractions, helping the hitherto tightly closed cervix to open and propelling the baby into the vagina during labour. About six weeks after pregnancy, the muscles have shrunk again and the uterus has returned to its normal size.