AnencephalyFolic AcidSpinal DysraphismSpina Bifida OccultaNeural Tube DefectsEncephalocelePregnancyCongenital AbnormalitiesFolic Acid DeficiencyFood, FortifiedPregnancy, TwinFetal DeathPreconception CareDiseases in TwinsAmniotic Band SyndromePrenatal CareInfant, NewbornDietary SupplementsFetal DiseasesAmniotic FluidPrenatal Diagnosis