LONDON — 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 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. 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.”

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.