OBJECTIVE: To assess maternal diet and nutritional adequacy prior to conception and during pregnancy. DESIGN: Follow-up of a cohort of pregnant women with collection of questionnaire data throughout pregnancy and after delivery. SETTING: Antenatal clinics at two public hospitals in Porto, Portugal. SUBJECTS: Two hundred and forty-nine pregnant women who reported a gestational age below 13 weeks at the time they attended their first antenatal visit. RESULTS: 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. CONCLUSION: 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.