The exact cause of preeclampsia is unknown. Experts believe it begins in the placenta — the organ that nourishes the fetus throughout pregnancy. Early in pregnancy, new blood vessels develop and evolve to efficiently send blood to the placenta. In women with preeclampsia, these blood vessels don't seem to develop properly. They're narrower than normal blood vessels and react differently to hormonal signaling, which limits the amount of blood that can flow through them.

Causes of this abnormal development may include:

  • Insufficient blood flow to the uterus
  • Damage to the blood vessels
  • A problem with the immune system
  • Certain genes

Other high blood pressure disorders during pregnancy

Preeclampsia is classified as one of four high blood pressure disorders that can occur during pregnancy. The other three are:

  • Gestational hypertension. Women with gestational hypertension have high blood pressure but no excess protein in their urine or other signs of organ damage. Some women with gestational hypertension eventually develop preeclampsia.
  • Chronic hypertension. Chronic hypertension is high blood pressure that was present before pregnancy or that occurs before 20 weeks of pregnancy. But because high blood pressure usually doesn't have symptoms, it may be hard to determine when it began.
  • Chronic hypertension with superimposed preeclampsia. This condition occurs in women who have chronic high blood pressure before pregnancy who then develop worsening high blood pressure and protein in the urine or other health complications during pregnancy.
Jul. 03, 2014

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