folic acid

As to the benefits of using folic acid - I realise that the number of births was 1.57 per 1000 births (of the 1.3 million studied) in Quebec from 1990-2005 were born with severe congenital heart defects.  This was reduced to 0.90 or 0.97 per 1000 births post fortification.  (They do site a reduction of 6% per year).  Now, let's say 2 babies out of every 1000 births had birth defects - so now only 1 baby out of every 1000 births has birth defects.  Does that mean that adding B12 (folic acid) to our diets will actually be much of a benefit?