• Folic acid;
  • Maternal report;
  • Periconceptional;
  • Plasma;
  • Pregnancy cohort


Birth cohorts typically measure plasma folate in midgestation, but effects of folic acid supplementation are sometimes specific to the periconceptional period. The relationship between midgestation plasma folate and periconceptional supplementation is not known. We compared plasma folate at week 18 of gestation with self-report use of supplements comtaining folic acid from before pregnancy to week 17 of gestation.

Methods and Results

The sample comprised 2911 women from The Norwegian Mother and Child Cohort Study. For women reporting continuous supplementation from gestational week −4 to 17 (N = 238), median plasma folate was 15.72 at week 18 (in nmol/L). This was about threefold higher than the median plasma folate of 5.67 for women reporting no supplementation from week −4 to 17 (N = 844), but only slightly higher than the median plasma folate of 13.34 for all women reporting supplementation in weeks 13–17 (N = 1158). Reported supplementation before week 8 was not associated with plasma folate at week 18, in an analysis that adjusted for continued supplementation after week 8.


Overall we found a strong and coherent relationship between self-reported folic acid use and plasma folate at week 18. We also found that plasma folate at week 18 did not reflect self-reported supplementation before week 8. For periconceptional supplementation per se, self-report data may offer a better measure.