The vagina receives sperm from the penis of the male during sexual intercourse. The sperm pass through the opening of the cervix into the uterus on their way to find an egg that has been expelled from an ovary. A fertilized egg travels down the fallopian tube and, after many cell divisions, embeds itself in the wall of the uterus. For the first eight weeks of its development it is called an embryo; thereafter, it is a fetus.
If no fertilization occurs, the lining of the uterus, or endometrium, is shed during menstruation. This occurs about every 28 days. The menstrual cycle is under the control of hormones produced by the pituitary gland in the brain.
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