Dear Bingley and WW: I searched for "incidence spina bifida"
and got a link to a big bungh of links about the problem.
It also says CDC collects data, but I couldn't find a tidy
easy to comprehend graph. I can't read well enough to wade through these. Perhaps you can find something useful at:
http://www.noah-health.org/english/illness/neuro/spinabif.html