CDC (centers for Disease control) have guidelines on their website that say folic acid is necessary for all women child-bearing age.
This indicates to me that there were many studies done on birth defects, and folic acid is necessary.
They wouldn't publish guidelines like that unless there was proof.
Don't forget that this doesn't stop your child from having issues.
I took vitamins, and my son ended up with a list of brain abnormalities.
They feel some of the are genetic, which is not anything that folic acid can prevent.
Retardation can be caused by a lot of things.
Catching illnesses when pregnant, birth defects from drugs, an injury from a fall and placenta issues.
Diabetes undetected in the mother, genetic disorders (gene mutations that have nothing to do with vitamins or illness).
Having a baby is a crapshoot. You do what you can to avoid issues and be preventative.
It doesn't cover all your risks. I got a CVS that was normal, was told my baby was normal,
and he is mentally and physically challenged at birth, no indications telling us he would be that way.