Preeclampsia is a multisystem inflammatory syndrome that is not well understood. It is defined as the onset of hypertension (blood pressure greater than 140/90) and proteinuria during the second half of pregnancy (greater than 20 weeks' gestation). While the condition can remain mild until delivery, it can also evolve rapidly into severe hypertension, proteinuria, and eclampsia or hemolysis, elevated liver enzymes, and low platelets (HELLP) syndrome, with risk of organ and systemic complications and maternal or fetal death. Even when preeclampsia does not proceed to HELLP syndrome or eclampsia, severe preeclampsia can lead to neurological and visual disturbances, epigastric or right upper quadrant pain, pulmonary edema, or cyanosis. The only curative treatment once preeclampsia develops is delivery, with obvious implications for the health of the infant when it occurs preterm. Systems for diagnosing and classifying the severity of disease vary across professional societies and organizations, including the American College of Obstetrics and Gynecology (ACOG), the American Society of Hypertension (ASH), and obstetrics and gynecology professional organizations in the United Kingdom, Canada, New Zealand, and Australia. ACOG defines severe preeclampsia as any case of preeclampsia that includes one or more of the following characteristics: severe hypertension (systolic at or above 160 mm Hg or diastolic at or above 110 mm Hg), severe proteinuria (ACOG: greater than 5 g/24 hours; ASH: greater than 3g/24 hours), severe oliguria (very low urine output), cerebral or visual disturbances (i.e., headache, blurry vision, scotomata), right upper quadrant pain, pulmonary edema or cyanosis, impaired liver function, thrombocytopenia, or fetal growth restriction. Other organizations include the timing of onset (