JOHN FOSTER / SCIENCE PHOTO LIBRARY JOHN FOSTER / SCIENCE PHOTO LIBRARY
Usually starting when a girl is in early puberty an egg cell (or sometimes more) ripens in her ovaries and enters a fallopian tube. During its journey down the tube over the next few days the uterine lining thickens to nourish it should it be fertilized. If it is not it disintegrates and the uterine lining begins to break down. For three to seven days the lining sheds in the form of blood that issues from the vagina as the menstrual flow. This is known as a menstrual period. After the period ends the cycle repeats. This cycle continues monthly though out a womans child-bearing years but ceases during times of pregnancy.
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