Dean Mark Schissel makes me smarter by emailing to say that I am working off of wrong and out-of-date information. It looks as though one of my fears is groundless: insurance companies are not likely to take genetic information about folic acid metabolism and use it to deny coverage because women with the genetic marker are more likely to have babies with neural tube defects like spina bifida. The apparent association found appears to be a "false positive":
[Because] the MTHFR variant... has an allele frequency of ~40%... many studies have shown a correlation with this marker of some disease with a p-value of <.05, the usual cut-off for significance.... [I]f you test 100 diseases or conditions, you'll expect to get 5 false positives.... In the case of neural tube defects, there was a LOT of research on the role of MTHFR since it would have been an easily treatable cause of a serious problem. Unfortunately, the association turned out NOT to be causal...
It is disappointing to learn that the interaction of low folic acid intake with the MTHFR variant does not look to be a likely cause of neural tube defects.
It would, as Mark said, have been a very easy to treat (and very cheap to treat) cause of a serious medical problem.