For such an old drug, I'm surprised that the FDA didn't classify it as Pregnancy Category D (don't take it) until 2013.
https://www.fda.gov/drugs/drug-safety-and-availability/fda-d...
https://www.fda.gov/drugs/drug-safety-and-availability/fda-d...
It says at the bottom that the other fetal risks have been known for a long time.
"Valproate products have long been known to increase the risk of serious birth defects, in particular, neural tube defects such as spina bifida. This risk is already described in detail in the drug labels for valproate products."