The effect of maternal folate intake on small-for-gestational-age (SGA) births remains inconclusive. The present study aimed to investigate the associations of maternal folate intake from diet and supplements with the risk of SGA births using data from a cross-sectional study in Shaanxi Province of Northwest China. A total of 7307 women who were within 12 months (median 3; 10th–90th percentile 0–7) after delivery were included. Two-level models were adopted to examine the associations of folate (dietary folate, supplemental folic acid and total folate) intake with the risk of SGA births and birth weight Z score, controlling for a minimum set of confounders that were identified in a directed acyclic graph. Results showed that a higher supplemental folic acid intake during the first trimester was negatively associated with the risk of SGA births (≤60 d v. non-use: OR 0·80; 95 % CI 0·66, 0·96; >60 d v. non-use: OR 0·78; 95 % CI 0·65, 0·94; Ptrend = 0·010; per 10-d increase: OR 0·97; 95 % CI 0·95, 0·99). A higher total folate intake during pregnancy was associated with a reduced risk of SGA births (highest tertile v. lowest tertile: OR 0·77; 95 % CI 0·64, 0·94; Ptrend = 0·010; per one-unit increase in the log-transformed value: OR 0·81; 95 % CI 0·69, 0·95). A similar pattern was observed for the birth weight Z score. Our study suggested that folic acid supplementation during the first trimester and a higher total folate intake during pregnancy were associated with a reduced risk of SGA births.

Folate deficiency during embryogenesis is an established risk factor for neural tube defects in the fetus. An adequate folate nutritional status is also important for normal fetal growth and brain development. The aim of the present research was to study the association between folic acid use of the mother during pregnancy and child behavioural development. Within a population-based cohort, we prospectively assessed folic acid supplement use during the first trimester by questionnaire. Child behavioural and emotional problems were assessed with the Child Behaviour Checklist at the age of 18 months in 4214 toddlers. Results showed that children of mothers who did not use folic acid supplements in the first trimester had a higher risk of total problems (OR 1·44; 95 % CI 1·12, 1·86). Folic acid supplement use protected both from internalising (OR of no supplement use 1·65; 95 % CI 1·24, 2·19) and externalising problems (OR 1·45; 95 % CI 1·17, 1·80), even when adjusted for maternal characteristics. Birth weight and size of the fetal head did not mediate the association between folic acid use and child behaviour. In conclusion, inadequate use of folic acid supplements during early pregnancy may be associated with a higher risk of behavioural problems in the offspring. Folic acid supplementation in early pregnancy, aimed to prevent neural tube defects, may also reduce mental health problems in children.