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Teen Girls' Health

Why We Get Periods

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By Jennifer Ashton, M.D., Ob-Gyn with Christine Larson
WebMD Feature by “The Body Scoop for Girls”

My patient Jolie had heard it all in health class and even from her mother. But she still wasn’t totally clear on why we get periods. So I gave her the CliffsNotes version.

  1. At puberty the brain starts sending chemical signals to your body, via estrogen and other hormones, saying it’s time to get down to the work of reproducing.
  2. Those hormones wake up your ovaries and uterus. You reach menarche—your first period. After that your brain sends chemical signals out roughly once a month, telling your body to get ready for a baby. Every month powerful hormones, including estrogen and progesterone (another important reproductive hormone), rise and fall in your system.
  3. For the first two weeks of your monthly cycle, your uterus builds up a nice cushy lining to nurture a fertilized egg. Estrogen and progesterone are both necessary to build up the lining. I tell my patients they can think of estrogen as the building blocks of the endometrium and progesterone as the cement that holds it all together. You usually need both, in balance, for a healthy reproductive cycle.
  4. In weeks two and three of your cycle, estrogen reaches its highest levels.
  5. In weeks three and four, the ovaries crank up progesterone production. Around week three (or more precisely, usually two weeks before you get a period), the ovaries release the egg. The timing of this egg release is very unpredictable in teens, so it’s basically impossible to time when you’re fertile or not fertile. That’s why the “timing” method of birth control does not work for teens.
  6. In week four, if the egg isn’t fertilized, the uterine lining is flushed out and you get your period.
  7. Repeat from Step 1.

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