Pregnant women suffering from epilepsy need to better informed about the risks of birth defects from treatments used to control their seizures, researchers said on Tuesday.

The risk of serious birth defects such as a small head or stunted growth from anti-epileptic drugs (AEDs) is about 2 to 3 percent but a study of pregnancy registers showed it can be much higher with an older treatment.

“These preliminary but very informative results clearly show that not all AEDs are equally safe for the exposed children of pregnant women of childbearing potential,” said Diego Wyszynski, of Boston University Schools of Medicine and Public Health.

In a study of pregnancy registers of 2,637 women in Britain he found that women taking lamotrigine, which is sold by pharmaceutical giant GlaxoSmithKline under the brand name Lamictal and Novartis AG’s Tegretol, or carbamazepine, the risk of serious birth defects of was 2.1 and 2.3 percent.

But it increased to 5.9 percent in women taking the generic drug valproic acid.

A North American pregnancy register of 3,400 women showed a 7.3 percent risk of birth defects with the generic drug and 2.8 percent with the other treatments.

“Whenever possible, physicians who treat girls and young women with this condition should consider offering only those drugs that pose the smallest risk of birth defects,” Wyszynski, who presented his finding at a medical meeting in Paris, added in a statement.

Epilepsy is a neurological disorder that affects about 25 million women worldwide. It makes people vulnerable to seizures and is caused by excessive local stimulation of the brain. It can strike at any age.

“These data are extremely important because they can help doctors make more informed treatment decisions. However more data are needed,” Wyszynski added.