At the top of the vagina is the cervix which is a ring of muscle separating the vagina from the uterus or womb. The cervix allows menstrual fluid to flow from the uterus into the vagina, and during sexual intercourse it allows semen from a man's penis to flow from the vagina into the uterus. The uterus is a very muscular and stretchy organ in which babies grow during pregnancy. When a woman is not pregnant, each month the lining of the uterus grows thicker, then breaks down, causing her to menstruate. Connected to the top of the uterus are two Fallopian tubes, on the left and right. These tubes join the uterus to the two ovaries. The ovaries produce ova, or egg cells, that are needed for reproduction. Each month, an ovum travels from one of the ovaries into a Fallopian tube. If, after the woman has had sexual intercourse with a man, the ovum fuses with a sperm and implants in the wall of the uterus, the woman will become pregnant. The ovaries also produce the female hormone estrogen which causes a person to grow into a woman and to feel like a woman.