Objective: To study the effect of periconceptional multivitamin supplementation on neural tube defects and other congenital abnormality entities.

Design: Randomised controlled trial of supplementation with multivitamins and trace elements.

Setting: Hungarian family planning programme.

Subjects: 4156 pregnancies with known outcome and 3713 infants evaluated in the eighth month of life.

Interventions: A single tablet of a multivitamin including 0.8 mg of folic acid or trace elements supplement daily for at least one month before conception and at least two months after conception.

Main outcome measures: Number of major and mild congenital abnormalities.

Results: The rate of all major congenital abnormalities was significantly lower in the group given vitamins than in the group given trace elements and this difference cannot be explained totally by the significant reduction of neural tube defects. The rate of major congenital abnormalities other than neural tube defects and genetic syndromes was 9.0/1000 in pregnancies with known outcome in the vitamin group and 16.6/1000 in the trace element group; relative risk 1.85 (95% confidence interval 1.02 to 3.38); difference, 7.6/1000. The rate of all major congenital abnormalities other than neural tube defects and genetic syndromes diagnosed up to the eighth month of life was 14.7/1000 informative pregnancies in the vitamin group and 28.3/1000 in the trace element group; relative risk 1.95 (1.23 to 3.09); difference, 13.6/1000. The rate of some congenital abnormalities was lower in the vitamin group than in the trace element group but the differences for each group of abnormalities were not significant.

Conclusions: Periconceptional multivitamin supplementation can reduce not only the rate of neural tube defects but also the rate of other major non-genetic syndromatic congenital abnormalities. Further studies are needed to differentiate the chance effect and vitamin dependent effect.