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During each menstrual cycle, an egg is released by one of the ovaries (ovulation). The egg travels into the fallopian tube. If fertilization is to occur, sperm ejaculated into the vagina must swim up the uterus and into the fallopian tube where one sperm penetrates the egg. The fertilized egg then travels into the uterus, where it embeds itself in the lining of the uterus (endometrium).