More than 10,000 pregnant women in France took Sanofi's anti-epilepsy drug Depakine between 2007 and 2014 although the risk of fetus malformation was known, according to a study quoted by French weekly Le Canard Enchaine.
The weekly reported that the French health ministry had delayed publication of the study, which the ministry denied.
The ministry confirmed in a statement that a study was done by national drug agency ANSM and health insurance agency CNAMTS but added that it had not been hidden from families and that a first part of the study would be presented on August 24.
The ministry said it would study measures that needed to be taken and that the full study and an action plan based on it would be made public in September.
Sanofi said the company was not aware of the data in the study and could not comment on it.
A Sanofi spokeswoman told Reuters that the firm had become aware of the risk of fetus malformation in the 1980s and that these had been spelled out in the drug's documentation.
She added that from around 2003, Sanofi became aware that the drug - based on sodium valproate - also impacted the fetus' neurological development and could lead to autism or learning difficulties.
She said these potential side effects had been communicated to health authorities, which updated the drug's documentation in 2006.
She specified that drug documentation needs to be approved by health authorities in France.
In a report published in February, French social affairs inspection agency IGAS criticised the slow response of French health authorities and Sanofi with regard to the risks related to Depakine and its derivatives.
IGAS estimates that between 2006 and 2014 some 425 to 450 babies suffered congenital birth defects or were stillborn following exposure to the drug.
It said the drug has been on the market since 1967 and that the risk of fetus malformation had been established since the early 1980s. Despite those risks, the drug continued to be prescribed because of its effectiveness against epileptic seizures and bipolar disorder.
In October, the Paris prosecutor launched a preliminary investigation into the authorisation and marketing of the drug.