High blood pressure during pregnancy has several distinct causes depending on when it develops and what’s driving it. Some women enter pregnancy with elevated blood pressure, while others develop it after the 20-week mark due to problems with how the placenta forms and functions. A reading of 140/90 mmHg or higher is the threshold for diagnosis, and roughly 1 in 10 pregnancies involves some form of hypertensive disorder.
Four Types With Different Causes
Not all pregnancy-related high blood pressure is the same. Clinicians recognize four distinct categories, each with a different origin and timeline.
Chronic hypertension exists before pregnancy, appears before 20 weeks of gestation, or persists after delivery. The causes are the same as in the general population: genetics, weight, diet, kidney function, and vascular health. Pregnancy doesn’t cause it, but it does complicate it.
Gestational hypertension is new-onset high blood pressure that appears after 20 weeks without signs of organ damage or protein in the urine. The exact trigger isn’t always clear, but it’s linked to how the body adapts (or fails to adapt) to the cardiovascular demands of pregnancy. Blood volume increases by nearly 50% during pregnancy, and some women’s blood vessels don’t relax enough to accommodate that extra flow.
Preeclampsia is the most clinically significant form. It involves new high blood pressure after 20 weeks along with protein spilling into the urine or signs that organs like the liver or kidneys are under stress. This condition has a specific biological origin rooted in the placenta.
Chronic hypertension with superimposed preeclampsia occurs when someone who already has high blood pressure develops new protein in the urine or sudden worsening that signals preeclampsia has layered on top of the existing condition.
How the Placenta Triggers Preeclampsia
The root cause of preeclampsia starts weeks before any symptoms appear. Early in pregnancy, the developing placenta sends specialized cells into the wall of the uterus to remodel the blood vessels that will supply it. These vessels, called spiral arteries, need to widen dramatically so the placenta receives a steady, low-pressure blood supply. In preeclampsia, this remodeling partially fails.
When those arteries stay narrow or only partially open, blood reaches the placenta in high-pressure pulses rather than a smooth flow. This damages placental tissue and starves it of consistent oxygen. The stressed placenta then releases substances into the mother’s bloodstream that cause widespread blood vessel constriction and inflammation. One key factor is an anti-angiogenic protein that directly raises maternal blood pressure by tightening blood vessels throughout the body. The result is a systemic problem that can affect the kidneys, liver, brain, and blood clotting system, even though the origin is local to the uterus.
This is why delivery of the placenta is the only definitive treatment for preeclampsia. Once the source of those circulating factors is removed, the process begins to resolve.
Who Is Most at Risk
Several factors increase the likelihood of developing high blood pressure during pregnancy, particularly preeclampsia.
- Higher body weight: Women who are overweight or obese at the start of pregnancy develop preeclampsia at roughly 3.7% compared to 2.3% in normal-weight women. Excess weight contributes to baseline inflammation and vascular stiffness, both of which make the cardiovascular system less adaptable.
- Family history: Having a mother or sister who had preeclampsia roughly doubles the risk, pointing to a genetic component in how the immune system interacts with the placenta.
- First pregnancy: The immune system encounters the placenta’s foreign proteins for the first time, making the remodeling process more likely to go wrong. Subsequent pregnancies with the same partner carry lower risk.
- Pre-existing conditions: Chronic kidney disease, diabetes, autoimmune disorders like lupus, and existing high blood pressure all raise the baseline risk substantially.
- Race and ethnicity: Black women face a disproportionately higher risk of preeclampsia, particularly when combined with higher body weight. This disparity reflects both biological factors and the cumulative effects of health inequities.
- Arterial stiffness: Research from the American Heart Association has identified that stiffer arteries early in pregnancy, measured by how quickly a pulse wave travels through the body, strongly predict who will go on to develop preeclampsia. This suggests that underlying vascular health before pregnancy plays a significant role.
What Happens to the Body
When blood pressure climbs during pregnancy, the effects extend well beyond the cardiovascular system. In gestational hypertension, the impact may be limited to elevated readings. But in preeclampsia, multiple organs can come under strain.
The kidneys are often the first to show signs. Damage to the tiny blood vessels in the kidneys allows protein to leak into the urine, which is why urine testing is a routine part of prenatal visits. The liver can also be affected, with enzyme levels rising to twice their normal range or higher in severe cases. Some women experience headaches, vision changes, or upper abdominal pain as the condition worsens. Severe preeclampsia is defined by blood pressure reaching 160/110 mmHg or higher, or by evidence of significant organ involvement.
The baby is affected too. Because the underlying problem in preeclampsia is poor blood flow to the placenta, the fetus may not receive adequate nutrition and oxygen. About half of early-onset fetal growth restriction cases occur alongside preeclampsia, and when preeclampsia develops early (before 34 weeks), more than 90% of those cases involve a baby that is smaller than expected. The placenta can also partially separate from the uterine wall, a dangerous complication called abruption.
Reducing the Risk
For women identified as high risk, low-dose aspirin (81 mg daily) started after 12 weeks of gestation is the most well-supported preventive measure. The U.S. Preventive Services Task Force recommends it for anyone with a history of preeclampsia, chronic hypertension, diabetes, kidney disease, or multiple risk factors. It works by reducing inflammation and improving blood flow to the placenta during the critical window when spiral artery remodeling is still occurring.
The World Health Organization also recommends calcium supplementation of 1.5 to 2 grams daily for pregnant women in populations with low dietary calcium intake. Calcium plays a role in blood vessel relaxation, and deficiency appears to contribute to the vascular dysfunction that drives hypertensive disorders.
Beyond supplements, the modifiable factors are the ones you’d expect: entering pregnancy at a healthy weight, staying physically active, and managing any pre-existing conditions like diabetes or kidney disease before conception. None of these eliminate the risk entirely, because much of what drives preeclampsia is rooted in placental biology and immune function that can’t be fully controlled.
Blood Pressure After Delivery
Many women assume that blood pressure returns to normal immediately after birth, but the opposite often happens. After the placenta is delivered, the hormones it produced that helped relax blood vessels are suddenly gone. This causes blood pressure to rise, typically peaking between days 3 and 7 after delivery. The risk of serious complications from high blood pressure is actually highest during the first one to six days postpartum.
Hypertension can also appear for the first time after delivery in women who had normal readings throughout pregnancy. This is why blood pressure monitoring in the days and weeks after birth matters as much as it does during pregnancy.
The long-term picture is equally important. A hypertensive disorder during pregnancy is one of the strongest predictors of cardiovascular disease later in life. Women who have had gestational hypertension or preeclampsia carry an increased risk of chronic high blood pressure, heart disease, and stroke that persists for decades. Annual blood pressure checks and attention to cardiovascular health become especially important from that point forward.