The uterus (womb) is a pear-shaped, muscular organ in a woman's lower abdomen. It's where babies develop. The inner lining of the womb is called the endometrium or endometrial layer. As a woman's menstrual cycle progresses, the endometrium thickens and becomes full of blood vessels. If an egg is fertilised, it implants in the endometrial layer where it receives nourishment and begins to develop into a baby. If the woman doesn't become pregnant, she'll have a flow (period). This happens when the thickened endometrial layer is shed. The menstrual cycle begins again.
Attribution: BUPA