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What is Preeclampsia?

What is preeclampsia?

Preeclampsia is one of the most common, serious disorders of pregnancy. It affects 2-8% of all pregnancies in the U.S, and typically begins with newly elevated blood pressure (readings above 140/90 on two or more occasions) sometime after 20 weeks of pregnancy.

(Women who enter pregnancy with chronic high blood pressure can also develop a condition known as superimposed preeclampsia.)

Other common symptoms include swelling, rapid weight gain of over 2 lbs. in a week, fatigue, and protein in the urine. But high blood pressure can also have no symptoms, which is why it’s sometimes known as the silent killer.

What are the risks associated with preeclampsia?

For the Mother:

Preeclampsia is notoriously unpredictable. With careful monitoring and management, it often proceeds uneventfully and resolves shortly after delivery. But not always.

Between 1-3% of women with preeclampsia in the United States develop seizures, and up to 15% develop HELLP (a complex and potentially deadly multi-organ syndrome involving Hemolysis, Elevated Liver enzymes, and Low Platelets).

Strokes, another possible complication, are thankfully rare. (Strokes affect less than .03% of all deliveries overall). That said, having hypertension or preeclampsia more than doubles a woman’s (still very low) risk of having a stroke.

For the Baby:

Preeclampsia raises the risk of growth restriction, stillbirth, neonatal death, and placental abruption, a life threatening condition in which the placenta prematurely detaches from the uterus.

As with maternal complications, many adverse outcomes for the baby are rare. Take the case of stillbirth, for example. It affects roughly 4 out of 1000 pregnancies. But it is more common among women with preeclampsia, rising to 6-10 out of 1000 pregnancies. (For unknown reasons, the risk of stillbirth is highest in women with preeclampsia in a second or later pregnancy.)

To avoid serious complications, many pregnancies affected by preeclampsia need to be induced early, making preeclampsia a leading cause of preterm delivery. In the U.S., it is responsible for a whopping 15% of all preterm deliveries.

What causes preeclampsia?

We still do not know for sure. But multiple lines of evidence now point to problems with the placenta early in pregnancy.

According to the current leading theory, preeclampsia develops in two stages.

To understand this theory, we need to quickly review normal placental development.

Early in pregnancy, the embryo grows an organ (the placenta) designed to nourish it with mom’s blood supply. The growth starts during embryonic development, when a group of cells branch off from the developing embryo to implant along the uterine wall. These cells then invade the lining of the uterus and expand the uterine spiral arteries.

uterine artery placenta formation what is preeclampsia
By Mikael HäggströmOwn work, Public Domain, Link

In the so-called first stage of preeclampsia, this expansion goes awry. The uterine spiral articles remain constricted. (Why this happens during some pregnancies but not others is poorly understood.)

Initially, the constriction causes no problems. During early pregnancy, a protective shell within the placenta prevents unfiltered blood from passing through to the developing baby. (Exposure to reactive oxygen in mom’s blood can harm the embryo’s rapidly developing organs.)

what is preeclampsia causes placenta formation
By Bonnie Urquhart GruenbergOwn work, CC BY-SA 4.0, Link

Then, as pregnancy advances, the shell dissolves. To nourish the fetus, more and more blood must cross the placenta. And now the constricted spiral arteries become a problem.

In response to inadequate blood flow, the placenta releases hormones and other factors into the mother’s bloodstream. This initiates the second stage of preeclampsia, in which these factors cause maternal high blood pressure and other organ problems.

Prevention and management

For managing preeclampsia, early detection and treatment is key. Your doctor will therefore closely track your blood pressure in the second half of pregnancy.

That said, preeclampsia can come on suddenly during pregnancy and even weeks after delivery. Sometimes a critical diagnosis is missed or delayed.  You are your body’s best advocate, so it’s important to know what to look for, and to not fall prey to some common misconceptions about this condition.


For women at high risk of preeclampsia, a new approach to prevention has recently been endorsed by the American College of Obstetricians and Gynecologists: Taking a daily baby aspirin starting at the end of your first trimester.

To learn more about aspirin therapy for preterm preeclampsia and who is eligible, click HERE.

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About Amy

Amy Kiefer is a researcher by training, and earned her Ph.D. from the University of Michigan. She currently lives in the Bay Area with her husband and three children where she writes about fertility, pregnancy, and breastfeeding. Check out her blog,, for more great evidence-based pregnancy and parenting info.


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