New study says that even a faintly high blood pressure late in pregnancy can increase the risk of an underweight or stillborn baby.

Women, 36 weeks pregnant, with borderline high blood pressure (prehypertension) had a 70 percent risk increase for underweight or stillborns compared with women of a healthy blood pressure. Even those who weren't prehypertensive but had their blood pressure rise slightly were still more likely to have small infants. The study showed that a 2 percent increased risk of a low birth rate occurred as each point of diastolic blood pressure increased.

The researchers studied data on 150,000 women with blood pressure under 140/90 mm Hg and who carried a single baby for 37 weeks and longer were included in the study. Of these women, 11 percent developed prehypertension. There were 2,400 infants who had a low birth weight and 194 were still born.

It is recommended that obese women "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 researchers found that a rising blood pressure was more significant even after accounting for the mother's age and weight, diabetes and smoking history.

"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. The study authors said prevention, versus treatment, is critical.

"We are worried about the global epidemic of obesity, since obesity has a strong association with maternal cardiovascular health and risk of prehypertension," Wikstrom said.

"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.

The U.S. Centers for Disease Control and Prevention found that underweight infants are more likely to have health problems than normal weight babies.

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