HELLP syndrome is caused by widespread damage to the lining of small blood vessels during pregnancy, triggered by a placenta that doesn’t develop properly. The name stands for its three hallmarks: hemolysis (destruction of red blood cells), elevated liver enzymes, and low platelet count. It affects roughly 0.4% of pregnancies globally and is closely related to preeclampsia, though it can appear even without classic signs of high blood pressure.
The Root Cause: A Poorly Developed Placenta
In a healthy pregnancy, the placenta sends cells deep into the wall of the uterus to remodel the blood vessels that supply it. These remodeled vessels widen and relax, allowing enough blood to flow to the growing fetus. In HELLP syndrome, this remodeling fails. The cells that should reshape those blood vessels either don’t invade deeply enough or don’t trigger the necessary changes. The result is a placenta that doesn’t get adequate blood flow, creating cycles of restricted oxygen followed by bursts of re-oxygenation, a pattern called ischemia-reperfusion injury.
This oxygen instability sets off a chain reaction. The oxygen-starved placenta releases substances that damage the inner lining of blood vessels throughout the mother’s body. At the same time, it floods her circulation with proteins that block new blood vessel growth. The combination turns a local placental problem into a bodywide crisis affecting the liver, blood, and kidneys.
How Blood Vessel Damage Creates the Three Hallmarks
Once the inner lining of blood vessels is damaged, platelets rush to the injured sites and begin clumping together. As more and more platelets are consumed patching these microscopic injuries, the overall platelet count drops, sometimes dramatically. The clumping platelets also release chemicals that cause blood vessels to constrict, worsening blood flow and creating more damage in a vicious cycle.
Red blood cells suffer next. As they squeeze through tiny blood vessels now clogged with clumps of platelets and clotting proteins, they physically shear apart. This mechanical destruction of red blood cells is called microangiopathic hemolytic anemia, and it’s the “H” in HELLP.
The liver takes a particularly hard hit. Its dense network of small blood vessels makes it especially vulnerable to this kind of widespread microvascular injury. Tiny blood clots block flow within the liver, causing patches of tissue to die. Liver enzymes leak into the bloodstream as cells are destroyed. In severe cases, blood can pool beneath the liver’s outer capsule, forming a hematoma. About 39% of women with HELLP develop these subcapsular liver hematomas, and in 0.5 to 2% of cases, the liver actually ruptures.
Genetic Factors That Increase Risk
HELLP syndrome has a genetic component that helps explain why some women develop it and others with preeclampsia do not. Research has linked the condition to a region on chromosome 12, and several specific gene variations significantly raise the odds.
One of the strongest associations involves a clotting gene mutation called factor V Leiden. Women carrying one copy of this mutation are roughly 4.5 times more likely to develop HELLP than women without it. A variation in a gene involved in processing the B vitamin folate also appears more often in women with HELLP, found in 45% of affected women compared to about 30% of those with severe preeclampsia alone or healthy pregnancies.
Genes controlling the immune system play a role too. A variation in the gene for a receptor called TLR-4, part of the body’s first-line immune defense, was found at its highest frequency in women who developed HELLP, with roughly four times the odds compared to controls. Another immune-related gene variation affects a receptor involved in programmed cell death. Women carrying certain versions of this gene have immune cells that are slower to shut down, meaning their immune system may mount a prolonged attack against the placental cells trying to remodel uterine blood vessels. This directly interferes with the placental development that, when it goes wrong, kicks off the entire syndrome.
Variations in genes that control blood vessel growth factors also matter. Certain versions of the gene for vascular endothelial growth factor (a protein critical for building new blood vessels) nearly quadrupled the risk of HELLP, independent of whether it was a first pregnancy or the mother’s age.
The Fetal Contribution
In some cases, the fetus itself contributes to the disease. When a baby inherits a defect in the enzymes that break down fatty acids inside cells, partially processed fat molecules can leak into the mother’s bloodstream. These toxic intermediates damage her liver and blood vessels, compounding the injury already caused by poor placental development. This overlap helps explain why HELLP can sometimes be difficult to distinguish from another rare pregnancy liver condition called acute fatty liver of pregnancy.
Symptoms and When HELLP Appears
HELLP syndrome typically develops before the due date, most often in the third trimester. However, 15 to 30% of cases first appear after delivery, usually within seven days of giving birth. This postpartum onset catches many women and providers off guard, since the pregnancy itself is already over.
The symptoms often mimic a stomach virus, which can delay diagnosis. Nausea, vomiting, and pain in the upper right abdomen or just below the breastbone are the most common complaints, reported in 30 to 90% of patients. Headache occurs in about a third to two-thirds of cases. Visual changes like blurriness or seeing spots affect 10 to 20% of women. Swelling that isn’t limited to the ankles, including puffiness around the eyes or in the hands, shows up in more than half of cases.
Perhaps most concerning: about 15% of women with HELLP do not have high blood pressure or protein in the urine, the two classic warning signs of preeclampsia. This means the condition can develop in women who appear to have relatively normal pregnancy checkups, making awareness of the more subtle symptoms especially important.
How HELLP Differs From Similar Conditions
Several serious pregnancy conditions can look like HELLP syndrome, and telling them apart matters because treatments differ. Acute fatty liver of pregnancy is the closest mimic. Both cause liver problems and can lower platelet counts. The key difference lies in blood clotting: acute fatty liver tends to cause more severe clotting dysfunction, while HELLP produces a more dramatic, progressive drop in platelet count over time. The underlying biology is also distinct. Acute fatty liver stems from a buildup of fat in liver cells due to enzyme deficiencies, while HELLP is driven by blood vessel lining damage that leads to bleeding and cell death within the liver.
What Happens During Treatment
Delivery is the only definitive treatment for HELLP syndrome, because removing the placenta stops the source of the damage. If the pregnancy is far enough along, delivery is typically initiated promptly. When HELLP develops earlier, the medical team weighs the risks of prematurity against the dangers of continuing the pregnancy with active organ damage.
Corticosteroids are sometimes given, both to help the baby’s lungs mature before an early delivery and in hopes of improving the mother’s platelet count. However, a systematic review of clinical trials found low-certainty evidence that steroids have little to no effect on whether a mother needs a platelet transfusion. Their primary proven benefit remains fetal lung development.
After delivery, most women see their lab values begin to normalize within 48 to 72 hours, though full recovery can take longer. Women who’ve had HELLP in one pregnancy face higher odds of developing it or preeclampsia in future pregnancies, making close monitoring in subsequent pregnancies essential.
Severe Complications
When HELLP progresses or goes unrecognized, it can lead to life-threatening complications. Liver rupture, though rare at 0.5 to 2% of cases, carries a maternal mortality rate of 17% and a fetal mortality rate of 38%. The widespread clotting that drives the syndrome can also spiral into disseminated intravascular coagulation, where the body’s clotting system essentially burns through its resources and can no longer stop bleeding anywhere. Placental abruption, kidney failure, and fluid in the lungs are additional risks that make early recognition critical.