AnencephalySpinal DysraphismSpina Bifida OccultaSpina Bifida CysticaNeural Tube DefectsMeningomyeloceleEncephaloceleFourth VentricleHydrocephalusPregnancyCongenital AbnormalitiesFolic AcidInfant, NewbornFetal DeathPrenatal DiagnosisDiseases in TwinsCerebrospinal Fluid ShuntsIndividuationFetal DiseasesPregnancy, TwinPrenatal CareFood, FortifiedUltrasonography, PrenatalPreconception CareAmniotic FluidAmniotic Band SyndromeUrinary Bladder, NeurogenicPregnancy Trimester, FirstAbnormalities, Severe Teratoid5,10-Methylenetetrahydrofolate Reductase (FADH2)Abortion, TherapeuticGestational AgeAbortion, InducedAbortion, SpontaneousAcrocallosal SyndromeNeural TubeCrown-Rump LengthPrevalenceArnold-Chiari Malformation