In truth, we don't if the taking of supplements is the reason why the rates of NTD dropped.
They dropped only slightly, this could be because of supplements, or because our nutrition has improved. The reason why we have small numbers of babies being born with NTD, is because of early screening and terminations.
My DD got pregnant, unplanned and was panicking, so we looked it up.
I took supplements, as far back as 1985 and through my pregnancy because of sickness.
Likewise I've taken Menopause Suppliments and they have made a difference to my hair and nails.
I felt a lot better post birth and breastfeeding when I started taking a multi mineral.
Our veg is really low in nutritional value, upto 70% lower than pre 1970, levels, so it's near impossible to eat the recommended levels.
We aren't funding research and statistic gathering, though, do we don't really know what levels are needed, to make a difference.