The uterus is thick walled, pear shaped body. It's 'cavity' is a potential one - in other words it can form a cavity if something is inside it, such as an embryo or fetus.
The fallopian tubes open into the uterus and carry the eggs (fertilized or not) into the uterine cavity.
The main part of the uterus is composed of involuntary smooth muscle which is capable of tremendous stretching and growth as pregnancy advances.
The lining of the uterus forms a specialized epithelial layer - the
endometrium - which undergoes changes during the menstrual cycle under hormonal
control.
If there is no conception the endometrium is shed during
menstruation.
The endometrium develops into the decidua - with more
pronounced glands and secretory activity and greater vascularity if conception
occurs. It is then ready to to sustain and support the blastocyst.