Abstract: Five trials involving 7391 women (2033 with a history of a pregnancy affected by a NTD and 5358 with no history of NTDs) were included. Four comparisons were made: 1) supplementation with any folate versus no intervention, placebo or other micronutrients without folate (five trials); 2) supplementation with folic acid alone versus no treatment or placebo (one trial); 3) supplementation with folate plus other micronutrients versus other micronutrients without folate (four trials); and 4) supplementation with folate plus other micronutrients versus the same other micronutrients without folate (two trials). The risk of bias of the trials was variable. Only one trial was considered to be at low risk of bias. The remaining studies lacked clarity regarding the randomisation method or whether the allocation to the intervention was concealed. All the participants were blinded to the intervention, though blinding was unclear for outcome assessors in the five trials. The results of the first comparison involving 6708 births with information on NTDs and other infant outcomes, show a protective effect of daily folic acid supplementation (alone or in combination with other vitamins and minerals) in preventing NTDs compared with no interventions/placebo or vitamins and minerals without folic acid (risk ratio (RR) 0.31, 95% confidence interval (CI) 0.17 to 0.58); five studies; 6708 births; high quality evidence). Only one study assessed the incidence of NTDs and showed no evidence of an effect (RR 0.07, 95% CI 0.00 to 1.32; 4862 births) although no events were found in the group that received folic acid. Folic acid had a significant protective effect for reoccurrence (RR 0.34, 95% CI 0.18 to 0.64); four studies; 1846 births). Subgroup analyses suggest that the positive effect of folic acid on NTD incidence and recurrence is not affected by the explored daily folic acid dosage (400 µg (0.4 mg) or higher) or whether folic acid is given alone or with other vitamins and minerals. These results are consistent across all four review comparisons. There is no evidence of any preventive or negative effects on cleft palate (RR 0.73, 95% CI 0.05 to 10.89; three studies; 5612 births; low quality evidence), cleft lip ((RR 0.79, 95% CI 0.14 to 4.36; three studies; 5612 births; low quality evidence), congenital cardiovascular defects (RR 0.57, 95% CI 0.24 to 1.33; three studies; 5612 births; low quality evidence), miscarriages (RR 1.10, 95% CI 0.94 to 1.28; five studies; 7391 pregnancies; moderate quality evidence) or any other birth defects (RR 0.94, 95% CI 0.53 to 1.66; three studies; 5612 births; low quality evidence). There were no included trials assessing the effects of this intervention on neonatal death, maternal blood folate or anaemia at term. Folic acid, alone or in combination with vitamins and minerals, prevents NTDs, but does not have a clear effect on other birth defects.

Plain-language summary: This review confirms that folic acid supplementation prevents the first and second time occurrence of NTDs and shows there is not enough evidence to determine if folic acid prevents other birth defects. Information about the safety of other current and alternative supplementation schemes and any possible effects on other outcomes for mothers and babies is also lacking. This review of five trials, involving 7391 pregnancies (2033 with a history of a pregnancy affected by a NTD and 5358 with no history of NTDs), shows the protective effect of daily folic acid supplementation in doses ranging from 0.36 mg (360 µg) to 4 mg (4000 µg) a day, with and without other vitamins and minerals, before conception and up to 12 weeks of pregnancy, for preventing the recurrence of these defects. There were insufficient data to evaluate the effects on other outcomes such as cleft lip and palate, miscarriages or any other birth defects. More research is needed on different types of supplementation programmes and the use of different types of supplements (such as 5-methyl-tetrahydrofolate -5-MTHF), particularly in countries where folic acid fortification of staple foods like wheat or maize flour is not mandatory and where the prevalence of NTDs is still high. The overall quality of the evidence for neonatal outcomes was high for NTDs, whereas, it was of low quality for other neonatal outcomes. The overall quality of the evidence for maternal outcomes was rated as moderate.