The 93% figure rests on six events in one small trial
Cochrane put the reduction at 93%, then reported an interval running from a 100% reduction to a 32% increase. It rests on one trial in which six defects occurred, all in one group.
What the study found
No trial has shown folic acid prevents a first neural tube defect
Cochrane put the reduction at 93%, then reported a confidence interval running from a 100% reduction to a 32% increase. It rests on one trial in which six defects occurred, all of them in the comparison group. The case for advising everyone rests on population data instead.
The caveat · One of five trials judged low-bias
An absence of proof is not proof of absence. Only five randomised trials of folate around conception exist at all, and just one was judged at low risk of bias. What this shows is that a question most people assume was settled by trial was put to a single small trial that could not settle it.
What we don't know
Whether folic acid prevents a first neural tube defect has never been put to a trial large enough to settle it. The one trial rests on six defects, all in one arm, and reviewers disagree on what that can carry.