Time trends in neural tube defects prevalence in relation to preventive strategies: an international studie. - Rosano,A.; Smithells,D.; Cacciani,L.; Botting,B.; Castilla,E.; Cornel,M.; Erickson,D.; Goujard,J.; Irgens,L.; Merlob,P.; Robert,E.; Siffel,C.; et al. (J Epidemiol Community Health, 1999)