French pharmaceutical giant Sanofi is being investigated for manslaughter amid a scandal over its epilepsy drug Depakine.
The firm is accused of being late to reveal the risk the medication poses to pregnant women.
Health authorities say when mothers-to-be took the drug they increased the chances of their children having birth defects, autism and learning difficulties.
Sanofi insists it had fulfilled its obligations in terms of informing people about the side-effects of Depakine
The medication, mainly composed of sodium valproate, is used to treat seizures but has also been widely employed as a mood stabiliser to help people with bipolar disorder.
The French drugs agency (ANSM) estimates that Depakine could be the cause of birth defects in 2,150 to 4,100 children and slow neurological development in 16,600 to 30,400, since 1967.
Today's development comes after an inquiry initiated by the organisation (APESAC), which represents alleged victims of the scandal.
Marine Martin, president and founder of APESAC, called it "a great victory for the families of victims of Depakine”.
Sanofi has already been indicted for aggravated fraud and unintentionally causing injury over the use of the drug.
According to the daily newspaper, Le Monde, the aim of the investigation is now to determine whether the pharmaceutical company can be held responsible for the death of four babies (in 1990, 1996, 2011 and 2014), whose mothers took Depakine during their pregnancy.
Last month, a French court found the state, Sanofi and doctors who prescribed the drug responsible in the case. It ordered the state to pay thousands of euros in compensation to families, saying officials didn't take the necessary measures to ensure the drug was not taken by pregnant women.