In a large investigation of more than 151 million pregnancies in the United States between 1970 and 2010, researchers discovered that the prevalence of high blood pressure among pregnant women soared by thirteenfold over that time period (from 0.11 to 1.52 percent). Scientists highlighted that the rate of chronic hypertension steadily edged upward with advancing maternal age. Also, African American women experienced more than double the prevalence of hypertension compared with white women (1.24 versus 0.53 percent). “It was pretty startling for us when we observed that the problem of blood pressure in pregnancy has risen so much for women in this country,” says lead investigator Cande V. Ananth, PhD, chief of the division of epidemiology and biostatistics in the department of obstetrics, gynecology, and reproductive sciences at Rutgers Health in New Brunswick, New Jersey. He and his colleagues based their findings on data from the National Hospital Discharge Survey, which represents all pregnant women during that time.
The Dangers of Elevated Blood Pressure
“Chronic hypertension can not only have profound health effects on the mother, but also on the baby. And the risks can go way beyond the period of pregnancy and post pregnancy,” says Dr. Ananth. The Centers for Disease Control and Prevention (CDC) estimates that 1 in every 12 to 17 pregnant women experience chronic elevated blood pressure. This may cause preeclampsia, (a condition of hypertension during pregnancy that can damage the liver and kidneys) or eclampsia (seizure or stroke that occurs in women with preeclampsia). A mother’s high blood pressure can increase the chances of miscarriage, a low birth weight, and preterm delivery, which can cause a baby to have lung problems and other long-term health issues, according to the March of Dimes. The mother’s high blood pressure makes it more difficult for the baby to get enough oxygen and nutrients to grow. Although gestational hypertension usually goes away after giving birth, some who are pregnant with hypertension continue to have hypertension in the future, which increases their likelihood for having a heart attack, stroke, and kidney failure, according to the CDC.
Having Children Later in Life Plays a Role
The report noted that, to a large degree, increasing numbers of older mothers are responsible for these high levels of hypertension. As people age, their blood vessels gradually lose some of their elastic quality and that promotes high blood pressure, according to the American Heart Association. Study figures from 2005 show that hypertension among young 20-year-old expecting women was 0.57 percent, compared with 1.47 percent among mothers-to-be in their late forties. “The age effect is one of the most prominent findings that we saw in this study,” says Ananth. “It was a huge risk factor.” He estimates that women today having children are four to five years older on average than they were in the 1970s. In addition, assisted reproductive methods, such as in vitro fertilization, may be compounding blood pressure problems among older mothers. “These procedures carry an implicit risk for many complications,” says Ananth. “We don’t know if chronic hypertension is one of them, but that’s something we need to explore.”
African Americans Face Greater Risks
Study authors also observed a substantial disparity between white and black expectant mothers. These results supported previous research, published in July 2018 in the Journal of the American Heart Association, which indicated that black adults are up to 2 times more likely to develop high blood pressure by age 55 compared with whites. This study also concluded that many of these racial differences develop before age 30. The greater risk among African Americans may be attributed to higher rates of obesity and diabetes, salt sensitivity, or differences in genes, according to the American Heart Association. Smoking, drug use, and greater social disadvantage may also be contributing factors. The researchers did remark, however, that the difference in high blood pressure prevalence between white and black women appears to be narrowing, as the rate rose 7 percent among white women versus 4 percent in African Americans over the study time period. “Since black women already had substantially higher rates of hypertension, it is not surprising that their rates did not rise as rapidly over time as white women,” says Sarah Samaan, MD, a cardiologist with Baylor Scott & White Legacy Heart Center in Plano, Texas, who was not involved in the investigation.
Some Puzzling Results and Areas for Future Research
Although obesity and smoking are proven risk factors for high blood pressure, this scientific team observed that they had no impact on the study results over the four decades. “We were stunned by this finding because we initially thought obesity and smoking would be explaining factors, but that didn’t pan out,” says Ananth. “It has left us scratching our heads, and we know we will have to look at this in greater detail in the projects to come.” Dr. Samaan warns that although the increase in risk was not driven by higher rates of obesity and smoking, these are still proven important risk factors that individuals can control. “For that reason, quitting smoking, a healthy diet, and a reasonable amount of exercise are important for all women, pregnant or not, to help reduce the likelihood of hypertension and other chronic diseases,” she says. Ananth also points out that the data analyzed here identified women with “high blood pressure” but did not provide actual blood pressure readings. “We lack the true clinical data in these women,” says Ananth. “Was a diagnosis of chronic hypertension made on a single spike in a reading or was there persistence in the blood pressure measurement? We are not sure.” Another consideration is the definition of hypertension, which has shifted over the years. Prior to 2017, the American Heart Association defined high blood pressure as a reading of 140/90 millimeters of mercury (mmHg) or higher, according to Harvard Health Publishing. But the latest guidelines have lowered the threshold, defining measures of 130–139/80–89 mmHg as high blood pressure and readings of 120–129/79-and-below mmHg as elevated blood pressure. “We’d like to standardize the blood pressure measurements so the definition of hypertension is consistent across the years,” says Ananth. “Right now they are not and this is a major limitation to our study.” He advises all women contemplating pregnancy to talk to their physician and get their latest blood pressure measurements. “If it is high, find out ways to reduce it and bring it to the normal levels,” says Ananth. “If you’re obese, think of ways to bring down body weight. If you smoke or have other unhealthy behaviors, consider reducing them or eliminating them.”
