To assess folic acid supplementation rates and validate the self-reporting of folic acid supplement use among pregnant women in a multiethnic cohort.

Secondary analysis of a prospective cohort study.

Self-reported folic acid supplement use in the Amsterdam Born Children and their Development study cohort was compared with serum folate concentrations using non-parametric trend analysis and linear and logistic regression.

A total of 4234 pregnant women of various ethnic backgrounds.

Serum folate levels showed a significant positive linear trend as reported use of folic acid increased (P < 0·001), which was supported by linear regression (r = 0·49). Odds of having low serum folate concentration decreased with reported early start of folic acid intake. Young, multiparous or non-Western women reported less pre-conception folic acid intake. Non-Western women showed lower serum folate concentrations. The overall rate of over-reporting, i.e. serum folate concentrations ≤20 nmol/l while reporting the use of folic acid supplements, was 20·7 %. Women of Surinamese and Moroccan ancestry had higher odds of over-reporting (OR = 2·3; 95 % CI 1·5, 3·5 and OR = 2·3; 95 % CI 1·3, 4·0, respectively). The odds for Surinamese women remained significant after adjusting for the onset of supplement use, parity and age (OR = 1·7; 95 % CI 1·1, 2·6).

Although self-reporting is a valid method for assessing folic acid supplement use in a multiethnic population, some participants do over-report. Surinamese and possibly Moroccan women appear to over-report more often. Rates of supplementation are low, especially in non-Western women. This suggests the need for intensifying current campaigns or perhaps even additional advice to start or continue to use folic acid post-conceptionally.