The mysterious high blood pressure disorder known as preeclampsia is unique to human pregnancy. It is uncommon — an estimated 5 percent to 7 percent of pregnancies — and as the Preeclampsia Foundation explains, most women will deliver healthy babies and fully recover.

Even rarer and less understood is HELLP syndrome, in which blood cells break down. HELLP is believed to be a severe form of preeclampsia, but the relationship between the disorders remains controversial.

Any pregnant woman can develop preeclampsia, but experts have identified some risk factors. The American College of Obstetricians and Gynecologists recommends high-risk women take low-dose aspirin, and says women with a few moderate risk factors should consider it.

High risk factors:

  • History of preeclampsia
  • Chronic hypertension outside of pregnancy
  • Diabetes or kidney disease
  • Autoimmune disease such as lupus
  • Multiple gestation (twins or more)

Moderate risk factors:

  • First pregnancy (excluding miscarriage and abortions)
  • Obesity (Body Mass Index over 30)
  • Family history of preeclampsia in a sister or mother
  • Age 35 or older