For the study, the researchers analyzed data from about 151 million hospitalizations between 1970 and 2010 to determine the rates of chronic high blood pressure in pregnant women aged 15 to 49.
Chronic high blood pressure was defined as high blood pressure before pregnancy or during the first 20 weeks of gestation. The researchers used a lower measurement -- 140/90 -- than the 130/80 benchmark that now brings a high blood pressure diagnosis.
High blood pressure poses risks for both fetus and mother. It increases the odds of stillbirth, as well as the mother's risk of preeclampsia (life-threatening high blood pressure during pregnancy), stroke, heart failure and heart muscle disease, kidney failure and death.
Nearly 1 million (0.63%) of the women had chronic high blood pressure during pregnancy, and the rate rose steeply over time -- from 0.11% in 1970 to 1.52% in 2010, a more than 13-fold increase, according to the report.
Since 1979, the rate increased an average of 6% every year, the study findings showed.
White women had a slightly higher annual increase (7%) than black women (4%), but black women were more than twice as likely to have high blood pressure during pregnancy.
The report was published Sept. 9 in the journal Hypertension.
"Women who already have high blood pressure and are planning to become pregnant should work closely with their health care provider to closely monitor and manage their blood pressure, especially during pregnancy, to reduce the serious health risks to both themselves and their unborn child," study author Cande Ananth said in a journal news release.
Ananth is chief of epidemiology and biostatistics in the department of obstetrics, gynecology and reproductive sciences at Rutgers Medical School in New Brunswick, N.J.
He noted that older mothers were more likely to have chronic high blood pressure.
"Since more women are electing to postpone their first pregnancies, and advanced maternal age is strongly associated with chronic high blood pressure, women should be aware of the risks associated with having high blood pressure during pregnancy," Ananth said.