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Women with prehypertension -- sometimes called borderline high blood pressure -- at 36 weeks of pregnancy had about 70 percent greater odds for low birth weight or stillbirth compared to women with normal blood pressure, the new Swedish study found.
But even mothers-to-be whose blood pressure rose in late pregnancy without becoming prehypertensive were more likely to have small babies, researchers said.
The researchers stressed, however, that their study showed only an association, not a cause-and- effect relationship, between blood pressure and fetal outcomes.
Infants with low birth weight are more likely to have health problems than normal-weight babies, according to the U.S. Centers for Disease Control and Prevention.
The study authors said prevention, rather than treatment, is key.
"We do not suggest treating women with medications, since earlier studies have not shown that this is beneficial to the mother or unborn child," said lead researcher Dr. Anna-Karin Wikstrom, an associate professor of obstetrics at Uppsala University in Sweden.
For the study, Wikstrom and colleagues collected data on more than 150,000 women listed in a Swedish obstetric database. Only women who carried their babies for 37 weeks or longer, whose blood pressure never rose above 140/90 millimeters of mercury (mm Hg) during pregnancy, and were having a single baby were included in the study.
Prehypertension is a systolic pressure (the top number) between 120-139 mm Hg or a diastolic pressure (the bottom number) between 80-89 mm Hg, or both.
About 11 percent of the women in the study developed prehypertension. Overall, more than 2,400 babies were born underweight and 194 were stillborn, the researchers found.
Woman whose diastolic blood pressure rose 15 points or more and developed prehypertension were more likely to have an underweight baby, Wikstrom said.
A rise in diastolic blood pressure that didn't reach prehypertension still increased the risk of low birth weight, with the likelihood rising 2 percent for every point, the researchers found.
Wikstrom suspects widespread obesity may be contributing to blood pressure increases. "We are worried about the global epidemic of obesity, since obesity has a strong association with maternal cardiovascular health and risk of prehypertension," she said.
Obese women who plan a pregnancy in the near future should "change their lifestyle in order to lose weight before conception to optimize their own health and the health of their fetus during pregnancy," Wikstrom said.
The report was published Feb. 1 online in the journal Hypertension.
"Women who have a rise in blood pressure should have close surveillance," said Dr. Jennifer Wu, an obstetrician and gynecologist at Lenox Hill Hospital in New York City.
Wu said she would consider inducing delivery early if the baby is in danger of being stillborn.
These women should have frequent ultrasound to look at blood flow and fluid levels and "catch any signs of growth restriction or deterioration of the health of the baby," she said.
Dr. James Ducey, director of maternal-fetal medicine at Staten Island University Hospital in New York City, agreed that these pregnancies need to be watched closely.
The first step is to see how well blood is flowing to the baby, Ducey said. "Once we see this rise in blood pressure, we should try to prevent the stillbirths," he said. "This might involve an earlier delivery."
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