An estimated 2,150 to 4,100 children in France suffered “severe malformations” in the womb between 1967 and 2016 after their mothers took a treatment against epilepsy and bipolar disorders known as valproate, France’s drug regulator said on Thursday.

Women who took the drug during pregnancy to treat epilepsy were four times more likely to give birth to babies with congenital malformations, said a preliminary study jointly issued by the French National Agency for the Safety of Medicines (ANSM) and the national health insurance administration.

“The study confirms the highly teratogenic” — that is, capable of causing birth defects — “nature of valproate,” said Mahmoud Zureik, scientific director of ANSM and co-author of the report.

The types of birth defects attributed to the drug included spina bifida — a condition in which the spinal cord does not form properly, and can protrude through the skin — as well as defects of the heart and genital organs.

The risk of autism and developmental problems was also found to be higher and will be quantified in a follow-up report due later this year.

An earlier estimate suggested that 30 to 40 percent of children exposed in the womb could suffer such disorders.

Valproate, which has been manufactured in France by Sanofi under the brand Depakine in the field of epilepsy and Depakote and Depamide in bipolar disorders, is also believed to cause slow neurological development.

ANSM, France’s drug authority, said it would also publish a study on neurological effects in the second half of 2017.

Sanofi was not immediately available for comment.

Valproate was approved as an anti-epileptic drug in France in 1967 and has become one of the most widely prescribed treatments for the condition worldwide.

“The risk of severe malformation is limited to the first two trimesters of pregnancy,” said Alain Weil, a researcher at the French health insurance administration and a co-author of the report.

The risk of birth defects associated with valproate has been known since the 1980s, especially for spina bifida, which occurs 20 times more frequently in fetuses exposes to the medication.

But the drug can still be prescribed to pregnant women when all other forms of treatment for epilepsy fail. That ruling, however, was only put in place in 2015.

Some families of children with birth defects born to women who took the drug while pregnant — grouped under an umbrella association known as APESAC — have sued the company, claiming that it did not adequately warn about the risks.