Blood pressure patterns during pregnancy predict later hypertension risk

Women with blood pressure levels in a range considered clinically normal during pregnancy but no mid-pregnancy drop in blood pressure face an increased risk of developing hypertension in the five years after giving birth, a new study shows.

These women, about 12 per cent of the population studied, would not be flagged as high-risk by current medical guidelines, but the new findings could help identify them as candidates for early intervention.

Researchers collected data on blood pressure and other health factors from 854 women during pregnancy and up to five years postpartum. That longitudinal approach allowed them to map the trajectory of women’s blood pressure throughout pregnancy and to spot a link between specific blood pressure patterns and hypertension several years later.

For most women in the study (80.2 per cent), systolic blood pressure remained low throughout pregnancy. In 7.4 per cent of women, blood pressure started out high, dropped during the second trimester, then increased again.

A third group of women (12.4 per cent) had slightly elevated systolic blood pressure that remained at a healthy level throughout pregnancy, but did not drop during the second trimester. Compared to the first group, these women faced a 4.91 times higher risk of hypertension in the five years after giving birth.

“This group of women would not be identified as having higher long-term hypertension risk by any of the current clinical criteria, since their blood pressure remained below diagnostic thresholds and most did not have other traditional risk factors,” said Shohreh Farzan, associate professor of population and public health sciences at the Keck School of Medicine of USC and the study’s senior author.

The findings suggest that tracking blood pressure patterns during pregnancy could help identify this underrecognised group and plan interventions that may prevent cardiovascular disease, said the study’s first author, Zhongzheng (Jason) Niu, a Presidential Sustainability Solutions Fellow at USC and an assistant professor of epidemiology and environmental health at the University at Buffalo.

Data for the study came from the Maternal and Developmental Risks from Environmental and Social Stressors (MADRES) cohort, a group of 854 low-income Hispanic women and their children who Keck School of Medicine researchers have worked with since 2015.

Researchers collected data on demographics, lifestyle factors and various health indicators periodically during pregnancy and at annual visits up to five years postpartum.

After collecting the data, researchers used a statistical approach known as latent class growth modeling to search for patterns in women’s blood pressure over the course of their pregnancies and to determine whether these patterns were associated with changes in their health at later visits.

They found three distinct patterns of systolic blood pressure during pregnancy: 80.2 per cent of women had consistently low blood pressure; 7.4 per cent of women had high blood pressure that dropped during the second trimester; and 12.4 per cent of women had slightly elevated blood pressure with no mid-pregnancy dip.

The group with high blood pressure and a mid-pregnancy dip had classic high-risk pregnancies, including more cases of preeclampsia and gestational hypertension than the other two groups. They also faced a 5.44 times higher risk of hypertension in the five years after giving birth, compared to the lowest-blood pressure group.

The 12.4 per cent of women with slightly elevated blood pressure but no mid-pregnancy dip faced a nearly equivalent risk of later hypertension — 4.91 times that of the lowest-blood pressure group.

“But because these women lacked traditional risk factors, they would not be alerted to their higher risk, nor would they be closely monitored for the development of high blood pressure,” Farzan said.

“Women’s health can change a lot between pregnancy and menopause, but it’s a period we know very little about,” Niu said.

“Our study helps fill that gap in understanding when it comes to hypertension.”

Simple changes in clinical practice can help identify and treat this high-risk group, Niu said. Clinicians can map blood pressure throughout pregnancy and follow up with women who lack a second-trimester dip, even if their readings stay within the normal range. The AHA, which updated hypertension cutoffs for the general population in 2017, may also consider adjusting its guidelines for blood pressure during pregnancy.

Additional studies are needed to replicate the findings and to learn more about women in both at-risk groups. Farzan, Niu and their colleagues are also exploring whether environmental exposures, including air pollution, heavy metals, and per- and polyfluoroalkyl substances (PFAS)—are linked to changes in blood pressure patterns during and after pregnancy.