In May 2022, the results of a long-running trial that focused on treating high blood pressure in pregnant women appeared in the New England Journal of Medicine. The multi-center Chronic Hypertension and Pregnancy (CHAP) project, which enrolled just over 2,400 women at 70 sites, including the University of Colorado, aimed to determine whether treatment of mild chronic hypertension during pregnancy would decrease complications related to the condition without compromising fetal growth.

The answer is yes, the authors concluded. Lowering the standard blood pressure threshold for initiating treatment from 160 mm Hg /105 mm Hg to 140 mm Hg /90 mm Hg “was associated with better pregnancy outcomes than a strategy of reserving treatment only for severe hypertension,” they wrote. The positive outcomes included lower incidences of complications for mother and fetus, such as the development of “preeclampsia with severe features” and the need for preterm birth prior to 35 weeks.

In addition, treating mild chronic hypertension with medications during pregnancy did not result in more babies being born with “small-for-gestational-age” birth weight, which had been the primary concern for administering the medications among professional organizations, including the American College of Obstetricians and Gynecologists (ACOG) and the American Society of Hypertension.

An important change in clinical care for pregnant women with hypertension

The CHAP study findings had far-reaching results, including spawning additional investigations now underway. More on that later. Most directly, the evidence moved ACOG to issue a new practice advisory on “clinical guidance” that recommended using the 140/90 blood pressure measurement as the “threshold” for initiating medical treatment of chronic high blood pressure during pregnancy.

In addition, ACOG recommended that those patients who were already on blood pressure medications prior to their pregnancy should continue rather than wait to see if the measurements reached a more severe range.

ACOG’s formal endorsement altered the landscape of obstetric practice, said Dr. Emily Su, a maternal-fetal medicine specialist with the University of Colorado School of Medicine. Su was the principal investigator for the CHAP study at the University of Colorado site.

The Colorado team enrolled patients from both UCHealth and Denver Health into the randomized clinical trial and also followed a cohort of control patients in an observational study, according to Jocelyn Phipers, Director of Perinatal Research Operations for Obstetrics & Gynecology at the University of Colorado.

CHAP study settles a longstanding question around high blood pressure during pregnancy

On the surface, a finding that anybody’s high blood pressure should be managed might seem unremarkable. The risks of uncontrolled chronic hypertension during pregnancy are considerable, Su said. But the question of whether pregnant individuals with “only” mild or moderate chronic hypertension should be treated went unanswered for years. The reason: that longstanding fear that administering medications to treat mild chronic hypertension in pregnancy could impair the mother’s blood flow to the placenta and uterus and subsequently lead to low birth weight babies.

The CHAP study’s conclusions also addressed – and allayed – those concerns, Su said. With evidence from a large, randomized clinical trial demonstrating that the benefits of treating mild to moderate hypertension outweigh the risks, providers now have reassurance about changing their practice. That’s a rare occurrence, Su noted.

“There aren’t many paradigm-shifting trials in obstetrics, so I think this one was monumental,” she said. “This study suggested there isn’t an overt concern for safety with the standard blood pressure medications that we use during pregnancy. At the same time, actively treating blood pressure to maintain [a level] below 140/90 also decreases the risk of significant complications.”

Clinicians readily adopt new standards for treating high blood pressure in pregnant women

The trial data could also serve to ease anxiety for pregnant women with hypertension, Phipers added.

“Before this trial, there was often a debate as to using medications because of unknowns,” she said. “Now [mothers] know they can stay on their medications.”

The move to adopt the ACOG recommendations did not cause a stir among faculty in the Department of Obstetrics and Gynecology at the University of Colorado School of Medicine, Phipers noted.

“Review of the literature and new recommendations are presented at division faculty meetings, and as a whole the division elects to move forward,” Phipers said.

Many new maternal health research avenues suggested by the CHAP study

For Su, the CHAP study spawns a number of important follow-up research possibilities. “Pregnancy can be thought of as a potential window into future maternal health,” she said. “Women who have high blood pressure and preeclampsia are more apt to develop cardiovascular disease later in life. The question now is whether [blood pressure] treatment in pregnancy affects [not only] long-term maternal [but also] child health,” she said.

The University of Alabama at Birmingham, the primary institution for the CHAP study, has received funding from the National Institutes of Health to conduct both a maternal and a pediatric follow-up study. Phipers said the University of Colorado is now enrolling patients in the maternal study and will start enrollment in the pediatric follow-up in 2024.

“These studies will provide investigators with greater insight into the long-term impact of tighter blood pressure control in pregnancy on both pregnant individuals and their offspring,” she said.

The CHAP maternal study will follow participants in the original CHAP trial for an additional 5 to 10 years, Su said. The aim: determine whether treating their mild hypertension during pregnancy mitigates the risk of long-term health issues.

The CHAP pediatric follow-up study will investigate the effects of blood pressure treatment on the offspring of the participants in the original trial.

“It’s now well-established that the in-utero environment a fetus experiences will affect his or her lifelong health,” Su said. “The question of the child follow-up study is whether treatment of mild maternal hypertension will impact offspring growth and neurodevelopment.”

Su said the opportunity to serve as principal site investigator for the CHAP study has spurred her in her work as a maternal-fetal medicine specialist and researcher.

“I feel privileged to have had the opportunity to participate in a trial that has been able to answer a question that has been long debated,” Su said. “The potential to contribute to ways to improve the health of pregnant individuals and their offspring gives me energy and inspires me to think about other research questions.”