High blood pressure can damage your heart in a number of ways, but for pregnant women, how does it affect the health of their babies? Researchers at Uppsala University in Sweden examined data from thousands of women throughout their pregnancies and found the outcomes for their babies revealed the dangers of gestational high blood pressure .

"Working as a clinical doctor in obstetrics, I often meet women with 'borderline high blood pressure,' and I wanted to find out if they had increased risks of adverse fetal outcomes," said the study’s lead author Dr. Anna-Karin Wikström, an associate professor of obstetrics at Uppsala University in Sweden, in a press release.

For the study, published in the journal Hypertension, Wikström and her research team analyzed medical records of more than 150,000 pregnant women between 2008 and 2014. They looked at the women’s blood pressure readings taken before 20 weeks of their pregnancies and 34 weeks after delivery. Out of all the participants, approximately 11 percent of the women developed prehypertension, which occurs during the early stages of high blood pressure. After the women gave birth, researchers tracked the health of the babies. Among all deliveries, 2,416 were born underweight while 194 were stillbirths.

When researchers compared blood pressure readings to birth outcomes, they found women who had prehypertension later in their pregnancies at 36 weeks were 69 percent more likely to give birth to a baby that was underweight and 70 percent more likely to have a stillbirth. They took into consideration the women’s age, weight, and whether or not they were smokers or diabetics to see if those factors had any effect on the outcomes, which they did not.

An individual’s blood pressure is read using two different numbers to gauge a person’s health. The top number measures the pressure in the arteries when the heart muscle contracts (beats inside the chest). The bottom number is the diastolic reading and the lower of the two values. It’s measured by the pressure in the arteries between beats, when the heart muscle is resting and refilling with blood. According to the American Heart Association , a normal systolic blood pressure is less than 120 millimeters of mercury (mmHg) and the diastolic level is less than 80 mmHg. Prehypertension is defined as 120 to 139 systolic over 80 to 89 diastolic. The first stage of hypertension is 140 to 159 over 90 to 99 and the second stage is 160 or higher systolic over 100 or higher diastolic.

Women who had an increase in 15 or more points in their diastolic blood pressure reading were more than twice as likely to deliver an underweight baby. The babies’ birth weights were 2.5 percent less than normal. Among all of the women, the chances of them giving birth to an underweight baby increased by 2 percent for each 1 point rise in the diastolic blood pressure. Researchers warn that there needs to be more investigation. However, having high blood pressure isn’t healthy regardless of whether or not you’re pregnant.

Source: Wikström AK, Gunnarsdottir J, Nelander M, Simic M, Stephansson O, and Cnattingius S. Prehypertension in late pregnancy linked with underweight newborns, stillbirths. Hypertension . 2016.