The uterus is a very special organ, shaped like an inverted pear, that sits deep in your belly. It's known as the "first home" because that's where a baby grows when someone becomes pregnant. During your cycle, it prepares every month with a very soft lining (the endometrium). When there is no baby, it releases this lining, which we call menstruation. It is a very strong and amazing muscle, capable of expanding many times its size!