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 no, 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.
Dr. 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 Department of Health and Human Services.
Dr. 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 medical records to look for birth defects. The scientists adjusted for potential confounders like diabetes and obesity.
The study, published online Tuesday in the journal BMJ, found similar rates of birth defects among children of pregnant women who took ACE inhibitors in their first trimester compared to women with untreated high blood pressure, those who took other blood pressure drugs, and those with normal blood pressure.
The numbers ranged from about five percent to eight percent and the differences could have been due to chance.
The researchers concluded that it was high blood pressure, not any medication, that was likely responsible for the problem.
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.
In the U.S., the Food and Drug Administration advises women to switch from ACE inhibitors to other drugs once they are pregnant, but there is no warning against using them, as there is for use in the second and third trimesters of pregnancy.
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.