All About the Female Reproductive System

9. Menstruation

If the egg is released during ovulation or the embryo fails to implant into the uterine wall, it is expelled along with the uterine lining, which consists of blood and tissues from the inner lining of the uterus. The monthly menstrual flow -- commonly referred to as the period -- occurs approximately every 28 days. Before the discharge begins, the female individual often experiences the physical discomforts of premenstrual syndrome (PMS), which include cramps, sore breasts, headaches, bloating, backaches, and fatigue. Other symptoms brought on by menstruation are emotional, such as depression and irritability. One period generally lasts about seven days on average.

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