The uturus is a peer shaped hollow muscular organ lying above the cervix. Looking from the side it is bent forward lying over the bladder. In about 20 % of women it is bent backward lying against the large bowel ( colon and rectum) and spine.
The cavity of the uterus is covered by a specialised lying. It is so important it got it's own scientific name. The scientific name is the endometrium.
The early pregnancy attach itself to the endometrium and than literally buries itself in the endometrium to obtain contact with the mother's blood vessels. See Early Pregnancy.
The endometrium's thickness is influenced by the female hormones estrogen and progesterone , and its response to the hormones causes menstruation.(See Normal Menstruation)