A large study of more than 2,400 pregnant women found that treating mildly elevated blood pressure reduces preterm births and preeclampsia, a high blood pressure condition that threatens the lives of mothers and babies.

The study, published in the New England Journal of Medicine, provides the first comprehensive data for treating non-severe forms of chronic hypertension during pregnancy. The Chronic Hypertension and Pregnancy or CHAP study concluded that treating mild chronic hypertension with medication is beneficial and safe for pregnant women and their babies.

The study took place at 70 academic medical centers and included pregnant patients at UW Health, UnityPoint Health-Meriter, Marshfield Clinic and Advocate Aurora Health. Kara Hoppe, DO, associate professor of obstetrics and gynecology, UW School of Medicine and Public Health, who led the Wisconsin sites of the study, said it may change guidelines on whether to treat women whose blood pressure is considered only mildly elevated.

“This is important additional evidence that treating chronic hypertension at a lower threshold leads to better outcomes for the mothers, and subsequently, their babies,’’ Hoppe said. “It also tells us that this treatment is safe for women.”

Current American College of Obstetricians and Gynecologists (ACOG) guidelines call for treating pregnant women ith severe high blood pressure, defined as pressures at or above 160/105. Generally, women with blood pressures less than 160/110 are not treated during pregnancy based on current ACOG guidelines. One concern has been uncertainty about whether medications could increase risk of babies born small for their gestational age.

The study screened more than 29,000 women who were 20-23 weeks pregnant. Those with severely high blood pressure received treatment and were not part of the study. About 2,400 women met criteria for mildly elevated blood pressure and were randomly assigned into two groups. One received no medication unless they developed severe blood pressure, as is current practice, and the other group was treated with standard hypertension medications: either labetalol, a beta blocker, or nifedipine, a calcium channel blocker.

Comparing the two groups, researchers found that the women and their babies in the medicated group had a reduced chance of developing serious conditions such as severe preeclampsia, placental abruption, death of the fetus or newborn, or complications that would require inducing birth before 35 weeks gestation. In the medicated group, 353 women (30.2%) developed these conditions compared with 427 (37%) in the untreated group. About 12.2% of the treated group, compared with 16.7% of the untreated group, had to deliver their babies before 35 weeks of gestation due to blood pressure issues. The birth weights of babies in the two groups were similar.

All races were represented in this trial and there were no different outcomes among groups. Study participants were 48% non-Hispanic black, 20% Hispanic and 28% white.

Hoppe, a co-author of the paper, is a fetal-maternal medicine specialist at UW Health who cares for women with high-risk pregnancies. She served as the site principal investigator for this trial.

Lead author Alan Tita, MD, PhD, of the University of Alabama-Birmingham, presented the results at the American College of Cardiology conference in Washington, D.C.