HELLP syndrome is a life-threatening condition that can potentially complicate pregnancy. It is named for 3 features of the condition: Hemolysis, Elevated Liver enzyme levels, and Low Platelet levels. It typically occurs in the last 3 months of pregnancy (the third trimester) but can also start soon after delivery. A wide range of non-specific symptoms may be present in women with HELLP syndrome. Symptoms may include fatigue; malaise; fluid retention and excess weight gain; headache; nausea and vomiting; pain in the upper right or middle of the abdomen; blurry vision; and rarely, nosebleed or seizures. Source: US NIH, NCATS
It affects around 0.5 to 0.9% of all pregnancies, i.e. many millions per year. About 70% of the cases develop before delivery, the majority between the 27th and 37th gestational weeks; the remainder within 48 hours after delivery.
The cause(s) of HELLP are currently unknown. though it's notedly more common in women who have preeclampsia or pregnancy induced hypertension (high blood pressure during pregnancy). Some women develop HELLP syndrome without showing signs of these conditions
More technical information : BMC Pregnancy & Childbirth
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