That's the finding of an eight year study conducted by the Medical Research Council of 1817 women who had produced an earlier baby with a neural tube defect.
The study found that 4 mg a day of folic acid had a 72 per cent protective effect on women who took it, compared to controls. In the group taking folic acid, the rate of neural tub defects was 1 per cent, compared with 3.5 per cent in the other two groups.
The results were so conclusive said the BMJ (27 July 1991), that the trial was stopped before the expected necessary 2000 pregnancies were obtained.
The chief medical office Sir Donald Acheson recommended that all women who had had an affected pregnancy should take one 5 mg tablet a day of folic acid supplements before starting another pregnancy and that all women who bear children should make sure an adequate amount is in their diet or in supplement form.