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"The reasons for the increase are not entirely clear," said study author Dr. Brian Bateman, an assistant professor of anesthesia at Harvard Medical School.
"We need to do more research to figure out which medicines are the best for insuring a good pregnancy outcome for both mother and baby," he said.
The findings are published in the October issue of Hypertension.
Bateman and his colleagues examined Medicaid claims from 2000 to 2007, looking for the records of women who had completed pregnancies. Of the more than 1 million women, nearly 48,500 (4.4 percent) took blood pressure medicines during pregnancy. From the start of the study to the end, the proportion of women taking the drugs increased from 3.5 percent to 4.9 percent.
The range of blood pressure medicines varied greatly, Bateman found. Often, women were on medicines other than methyldopa (Aldomet) or labetalol (Normodyne, Trandate), two drugs that are typically recommended during pregnancies.
Other medicines, including ACE inhibitors, should not be used during pregnancy, according to the American Congress of Obstetricians and Gynecologists, due to possible hazards to the developing fetus.
For all of the blood pressure medicines, 1.9 percent of the woman took them during the first trimester, 1.7 percent during the second trimester and 3.2 percent during the third trimester. For ACE inhibitors, 4.9 percent of those women took these during the second trimester and 1.1 percent in the third trimester.
Bateman found that women on blood pressure medicines tended to be older than those not on the drugs. They were more likely to be white or black compared to other ethnicities. They were more likely than nonusers to have diabetes and kidney disease.
While Bateman's study didn't look at why the number of women taking blood pressure drugs is on the rise, he speculated that the obesity epidemic and women delaying childbirth until they are older (and at more risk of getting high blood pressure) may explain the increase.
Older maternal age may indeed explain much of the increase, according to Dr. Suzanne Steinbaum, director of women and heart disease at the Heart and Vascular Institute of Lenox Hill Hospital, in New York City.
"When I look at my [pregnant] patients, they are not obese, just a little older," she said. "We are looking at a different group of women than what once was -- women who are older and maybe sicker and having babies."
High blood pressure in pregnancy definitely needs to be treated, Steinbaum said. However, some of the medications are dangerous to the baby. Methyldopa and labetalol are viewed as safest, Steinbaum agreed. "There's a safety record [with those]," she said.
Her advice? "If you are thinking of getting pregnant and you have high blood pressure and you take medication, talk to your doctor. You might be on a medicine that is not safe [during pregnancy]."
"I think this has been little studied," she said of the safety of blood pressure drugs during pregnancy.
The U.S. National Institutes of Health and the Agency for Healthcare Research and Quality funded the research.
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