Every woman knows the price to being the fairer sex: cramps… horrible, no good, very bad cramps. Menstrual cramping can certainly be a huge pain to deal with—literally—but have you ever wondered what causes them in the first place? Basically, the menstrual cycle comes with a side effect called “primary dysmenorrhea,” marked by the recurrent cramping pain that occurs in the lower abdomen during menses. When an egg doesn’t implant, the uterus sheds its excess lining and blood flow decreases as a result of all the contractions. Decreased blood flow means tissues don’t get enough oxygen, and voilà, your painful cramps are born.

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