Dear WW: I would doubt that there would be any likelihood
of a change in incidence of spina bifida. A lot of congenital anomalies may be associated with mother having virus infection such as rubella during third month of pregnancy. I don't recall reading that spina bifida is in that group. Nor have I read that it is familial. But I haven't read much about it.
Remember that many "changes" in incident of any disease
may be just a change in recognition and reporting of diseases.