Borderline High Blood Pressure and Pregnancy

Research shows blood pressure may not have to rise as high as previously thought to have negative outcomes during pregnancy. Here's what you can do about it.

pregnant woman getting blood pressure taken
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You may have heard that high blood pressure in pregnancy is dangerous and can lead to a serious complication called preeclampsia, which puts baby at risk for low birth weight and even stillbirth. But, it turns out that even if blood pressure doesn't get quite that high, it is still associated with those same negative outcomes.

A 2016 study published in the American Heart Association's journal Hypertension found that pregnant women with "prehypertention," or borderline high blood pressure, were also at risk for low birth weight and stillbirth.

Prehypertension Risks

Researchers in Sweden analyzed records of more than 150,000 pregnant women to see if the 11 percent who developed borderline high blood pressure had a higher incidence of bad outcomes for baby. Turns out, they did.

"In this study we showed that women with prehypertension in late pregnancy have higher risks of giving birth to a baby that is underweight or stillborn," lead study author Anna-Karin Wikström, M.D., Ph.D., an associate professor of obstetrics at Uppsala University in Sweden. "Their risks are about 70 percent higher for both complications," compared to women with normal blood pressure.

Among all the subjects, there were 2,416 small-for-gestational-age babies and 194 stillbirths.

In addition, the researchers found that the more mom's blood pressure increased from early to late pregnancy, the greater the risk of low birthweight—even if her blood pressure never reached prehypertensive levels.

"We found the higher her blood pressure increased during her pregnancy, the higher risk she had to give birth to a baby that is underweight—irrespective of the level of blood pressure in late pregnancy," Dr. Wikström says. Blood pressure affects the placenta, which supplies blood and nutrients to the growing baby—so an increase in blood pressure could prevent the placenta from getting everything it needs, resulting in a smaller infant.

Blood pressure measures the contracting of your heart (the top number) and the relaxing of your heart (the bottom number). Hypertension, or high blood pressure, is 140/90 or above, and prehypertension is 120-139 for the top number, 80-89 for the bottom, or both.

"Earlier studies have shown that women who develop hypertension during pregnancy have increased risks of give birth to a baby that is underweight or stillborn," Dr. Wikström says. But this is the first large study of prehypertension's effects in pregnancy. "The findings give further insight in the mechanism behind fetal growth, and suggest that maternal cardiovascular health is important for fetal growth," Dr. Wikström says.

Reducing Borderline High Blood Pressure

Borderline high blood pressure is fairly common, but doctors might not really be concerned because it doesn't cross the threshold for hypertension. Still, this study's findings highlight the importance of mom's heart health in pregnancy, and even before.

"We are worried about the global epidemic of obesity, since obesity has a strong association with maternal cardiovascular health and risk of prehypertension," Dr. Wikström says. "Therefore we suggest obese women who are planning a pregnancy in the near future to change her lifestyle in order to lose weight before conception, to optimize her own and her fetus' health during pregnancy."

Is there anything you can do to reduce your risk of prehypertension if you're already pregnant?

"We advise women who are already pregnant to keep their weight gain within the guidelines, since excessive weight gain during pregnancy is also associated with higher blood pressure elevation during pregnancy," Dr. Wikström says. Regular prenatal care is crucial as well. "Women who have a blood pressure in the prehypertensive level usually do not have symptoms, so we advise all pregnant women to go to their regular blood pressure checkups."

In addition to maintaining a healthy weight before and during pregnancy, there are a few other things you can do to try to keep your blood pressure from rising:

  • Drink lots of water (eight glasses a day)
  • Limit your salt intake
  • Get enough rest
  • Exercise (with your doctor's OK)
  • Elevate your feet a few times a day
  • Lie on your left side to reduce pressure on your major blood vessels
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