This is the part of the folate story a trial actually proved
Among women who had already had a pregnancy affected by a neural tube defect, 4mg of folic acid daily from before conception left 6 defects against 21 without it.
What the study found
Folic acid cut a repeat neural tube defect by 72%
This is the part of the folate story a randomised trial actually proved. Among women who had already had a pregnancy affected by a neural tube defect, a spine or skull that fails to close, taking 4mg of folic acid daily from before conception left 6 defects against 21 without it.
The caveat · Stopped early, which can inflate it
The 4mg dose tested here is ten times what the general population is now told to take. The trial was stopped early once the result crossed its stopping rule, which tends to make an effect look larger than it is. Twenty-seven defects occurred in the whole trial, so the estimate is rough. Every woman enrolled had already had an affected pregnancy, so this says nothing about a first one.