Skip to main content

A new study suggests that high blood pressure during early pregnancy is what raises the risk of major birth defects – not the medicines used to control the condition, as previously thought.

Pregnant women have been warned for years to avoid drugs called ACE inhibitors during the later stages of pregnancy to avoid the possibility of birth defects. But whether it was safe to take them during the first trimester wasn't clear.

A 2006 paper concluded it was not safe, and two later studies found an increased risk with other blood-pressure drugs as well.

Researchers behind a new, larger study suggest it's the high blood pressure itself that is responsible for the higher risk of birth defects, not the medications.

Compared to women without high blood pressure, those with the condition were more likely to have babies with congenital heart, brain or spinal-cord defects regardless of whether they were taking ACE inhibitors, other medications, or getting no treatment at all, the study found. The study was published online this week in the journal BMJ.

De-Kun Li and colleagues from Kaiser Permanente in California examined data covering more than 460,000 pregnant women and their babies from 1995 to 2008 for the study, which was paid for by groups including the U.S. Agency for Healthcare Research and Quality and the U.S. Department of Health and Human Services.

James Walker, a spokesman for Britain's Royal College of Obstetricians and Gynaecologists who was not linked to the research, said it was reassuring that blood-pressure drugs taken in early pregnancy probably weren't raising the risk of birth defects.

He said ACE inhibitors are not commonly prescribed to pregnant women because of past concerns, and that the main worry has been getting them off the drugs as soon as they are pregnant.

"What this study does is reassure us women can stay on the drug until they're pregnant and then stop," he said. "You never know how long someone is going to take to get pregnant and if they come off a blood-pressure drug for too long, it could be bad for them."

ACE inhibitors are used mainly to treat high blood pressure and heart failure. They work by relaxing the blood vessels and improving blood flow.

The researchers looked at pharmacy databases to see whether the women took any blood-pressure drugs during their pregnancy and at medical records to look for birth defects. The scientists adjusted for potential confounders such as diabetes and obesity.

Since earlier studies raised questions about the safety of taking ACE inhibitors once they were pregnant, it's unclear how many women are still commonly prescribed the drugs, among the most popular blood-pressure medicines.

Scientists aren't sure why high blood pressure in pregnant women could result in birth defects, but suggest there could be physiological changes in mothers that affect fetal growth.