To test the hypothesis that a concise intervention to promote the preconception use of folic acid (FA) supplements among mothers who visit a well-baby clinic (WBC) for the 6-month check-up of their youngest child is effective. Effectiveness was measured as intention to use or actual use of FA supplements before a next pregnancy among women who expected to be pregnant within 0–12 months.

Controlled intervention study with independent samples of intervention and control mothers. The intervention took place at the 6-month visit. A post-intervention measurement was done in the intervention group and a comparable measurement in the control group at the 11-month check-up visit.

The intervention, verbal and in writing, was implemented in four Dutch WBC and given by the WBC physician to the mothers who visited the WBC.

All mothers visiting the WBC were eligible for inclusion, unless they were unable to complete a questionnaire. The intervention group consisted of 198 (68 %) mothers recruited from 291 6-month intervention visits and the control group of 215 (84 %) mothers recruited from 255 11-month normal visits.

In mothers who expected to be pregnant within 0–12 months, the proportion using or intending to use FA was 65 % in the intervention group (n 49) v. 42 % in the control group (n 43; difference 23 %, 95 % CI 4, 43 %, P<0·05).

Health education intervention at the 6-month WBC visit is an effective means to promote the use of FA supplements or the intention to do so.

To assess maternal diet and nutritional adequacy prior to conception and during pregnancy.

Follow-up of a cohort of pregnant women with collection of questionnaire data throughout pregnancy and after delivery.

Antenatal clinics at two public hospitals in Porto, Portugal.

Two hundred and forty-nine pregnant women who reported a gestational age below 13 weeks at the time they attended their first antenatal visit.

Intakes of energy and macronutrients were within recommended levels for most women. Pregnancy was accompanied by increases in the dietary intake of vitamins A and E, riboflavin, folate, Ca and Mg, but declines in the intake of alcohol and caffeine. The micronutrients with higher inadequacy prevalences prior to pregnancy were vitamin E (83 %), folate (58 %) and Mg (19 %). These three micronutrients, together with Fe, were also those with the highest inadequacy prevalences during pregnancy (91 %, 88 %, 73 % and 21 %, respectively, for folate, Fe, vitamin E and Mg). Ninety-seven per cent of the women reported taking supplements of folic acid during the first trimester, but the median gestational age at initiation was 6·5 (interquartile range 5, 9) weeks. Self-reported prevalences of Fe and Mg supplementation were high, and increased throughout pregnancy.

The study identified low dietary intakes of vitamin E, folate and Mg both in the preconceptional period and during pregnancy, and low intake of Fe during pregnancy only. The low dietary intake of folate and the late initiation of supplementation indicate that current national guidelines are unlikely to be effective in preventing neural tube defects.