What is high blood pressure?

Blood pressure refers to how hard blood pushes against the artery walls when your heart beats. A certain amount of pressure is necessary for blood to move around your body, but too much pressure can cause serious problems.

Blood pressure is measured in millimeters of mercury (mmHg), and the measurement has two numbers: The top (systolic) number is the pressure when the heart contracts and pumps blood, and the bottom (diastolic) number is when the heart relaxes and fills with blood. When people talk about blood pressure readings, they say, "120 over 80," for example.

High blood pressure (hypertension) during pregnancy is defined as a reading of 140/90 or higher, even if just one number is elevated. Severe high blood pressure is 160/110 or higher.

Most women with high blood pressure can have a normal pregnancy. But having high blood pressure during pregnancy makes it more likely that you and your baby will have certain complications.

What is the difference between chronic high blood pressure and gestational hypertension?

Sometimes it's hard to know whether you have chronic high blood pressure or gestational hypertension until after you deliver your baby. If you have high blood pressure before you're 20 weeks pregnant, it usually means you have chronic high blood pressure. This is a long-term type of high blood pressure that affects up to 5 percent of pregnant women.

Gestational hypertension – also called pregnancy-induced hypertension – is high blood pressure that usually occurs for the first time at 20 weeks of pregnancy or later. Gestational hypertension is temporary and almost always goes away after childbirth.

What are the risk factors for chronic high blood pressure?

The risk factors that make chronic high blood pressure more likely include:

  • Age (The older you are, the higher the risk.)
  • Having a family history of high blood pressure
  • Being African American
  • Having diabetes or kidney disease
  • Having had preeclampsia in a previous pregnancy
  • Being overweight
  • Being sedentary
  • Smoking cigarettes
  • Eating too much salt
  • Drinking more than two alcoholic drinks a day
  • Poor nutrition, especially a diet that lacks fruits and vegetables

Some risk factors can't be changed, such as your genes or having certain health conditions. But you can make changes to reduce other risk factors, and these are important to know about when you're trying to get pregnant.

What are the symptoms of high blood pressure?

High blood pressure doesn't usually cause any symptoms, so you may not be aware of it unless your blood pressure is measured. High blood pressure can sometimes cause:

  • Severe headaches
  • Severe anxiety
  • Shortness of breath
  • Nosebleeds

How is high blood pressure diagnosed?

You'll have your blood pressure measured at each appointment during pregnancy. Your healthcare provider may also ask you to monitor your blood pressure at home.

High blood pressure is diagnosed when either or both the systolic or diastolic blood pressure is at an unhealthy level. Because blood pressure varies throughout the day, your provider may take several readings at different times.

A blood pressure reading of:

  • Less than 120/80 is considered healthy blood pressure (normal)
  • At least 120/80 but less than 140/90 is considered at risk for high blood pressure (prehypertension)
  • At least 140/90 but less than 160/110 is mild high blood pressure (mild hypertension)
  • 160/110 and above is severe high blood pressure (severe hypertension)

How does high blood pressure affect pregnancy?

Many women with mild high blood pressure have a normal pregnancy. Blood pressure typically falls slightly early in pregnancy and then returns to pre-pregnancy levels in the third trimester.

But the more severe high blood pressure is during pregnancy, the greater the risk of problems. There's also a greater risk of complications if you've had high blood pressure for a long time and it's damaged your heart, kidneys, or other organs. The risks are also higher for women with high blood pressure that's caused by another medical condition, such as diabetes or kidney disease.

High blood pressure risks include:

  • Preeclampsia: If high blood pressure develops after 20 weeks of pregnancy and you have protein in your urine, or if there are signs that certain organs aren't working properly (like your liver), this could signal a serious condition called preeclampsia. Preeclampsia that develops when you already have high blood pressure is called "superimposed preeclampsia." Between 13 and 40 percent of women with high blood pressure develop superimposed preeclampsia during pregnancy.
  • Having a baby that's smaller than normal: High blood pressure can mean that your baby doesn't get all the necessary nutrients and grows more slowly than usual (intrauterine growth restriction or IUGR). The risk of a baby being born small depends on how severe your high blood pressure is and whether you have other complications, like anemia or kidney disease.
  • Cesarean birth: Women with high blood pressure are at a higher risk of delivering by cesarean section (c-section), which is surgery that can lead to other complications. On average, about 4 out of 10 women with high blood pressure give birth by c-section.
  • Placental abruption: In this condition, part or all of the placenta separates from the uterine wall before the baby is born. There are different degrees of placental abruption, and in severe cases, a baby may not get enough oxygen and need to be born right away. If high blood pressure is mild, the risk of placental abruption is low, affecting about 1 in 100 women. If high blood pressure is severe or preeclampsia develops, the risk increases to range from 5 to 10 percent.
  • Preterm birth: If complications develop, or it looks like your baby isn't growing well, it may be necessary to deliver early. The more severe high blood pressure is, the more likely it is to need an early delivery. On average, 28 percent of women with high blood pressure have their baby before they're 37 weeks pregnant. Studies show between 62 and 70 percent of women with severe high blood pressure deliver early.

What can I do to have a healthy pregnancy?

Discuss your plans with your healthcare provider at a preconception visit. Some antihypertensive drugs, such as ACE inhibitors, raise a baby's risk of developmental problems if they're taken during pregnancy.

But don't stop taking your blood pressure medication during pregnancy unless your provider tells you to. Your provider will look for alternative medications to help keep your blood pressure under control.

If your high blood pressure is mild and you don't have other complications (such as diabetes or kidney disease), your provider may advise you to stop taking your blood pressure medication or to reduce the dose. Being off medication temporarily is unlikely to cause problems if your condition is mild, though it's possible you'll take medication again later in pregnancy.

Whether high blood pressure is mild or severe, keep all your prenatal appointments, so your provider can monitor you and your baby and spot any problems, such as rising blood pressure, signs of preeclampsia, or poor fetal growth as early as possible.

Also, try to maintain a healthy weight and diet. Talk to your provider about how much weight to gain during pregnancy.

Ask your provider if it's safe to exercise. In most cases you can continue to exercise, though your provider may recommend modifying the types of activities you do.

If you drink alcohol or smoke, it's important to stop to protect your health and your baby's.

When should I call my healthcare provider?

If you're checking your blood pressure at home, contact your provider if your blood pressure is above a certain level. Your provider can tell you what those levels should be and what to do if you go above them.

Keep a daily kick count of your baby's movements, and let your provider know immediately if you notice any changes. Also call your provider right away if you have:

  • A headache that's severe or doesn't go away
  • Swelling that increases dramatically over a few days. Some swelling during pregnancy is normal, but big changes are cause for concern.
  • Vision changes, including double vision, blurriness, seeing spots or flashing lights, light sensitivity, or temporary loss of vision
  • Intense pain or tenderness in the upper abdomen
  • Nausea or vomiting (other than morning sickness in early pregnancy)

Visit the Society for Maternal-Fetal Medicine's website for more information and to find an MFM specialist near you.