Nuchal Translucency MeasurementNeckPregnancy Trimester, FirstCrown-Rump LengthDown SyndromeUltrasonography, PrenatalChromosome DisordersFetal DiseasesPregnancy, High-RiskPregnancyChorionic Gonadotropin, beta Subunit, HumanPregnancy Trimester, SecondMedical Laboratory ScienceMaternal AgeGestational AgeKaryotypingChromosome AberrationsPregnancy OutcomeChorionic Villi SamplingMass ScreeningPregnancy-Associated Plasma Protein-ASensitivity and SpecificityTrisomyProspective StudiesReproducibility of ResultsNasal BoneFetal HeartPrenatal DiagnosisHeart Defects, CongenitalLymphangioma, CysticAmniocentesisChromosomes, Human, Pair 18AneuploidyFetal DeathSpina Bifida CysticaHeart Rate, FetalFetusFalse Positive ReactionsCongenital AbnormalitiesUrachal CystFetal ViabilityEstriolChromosomes, Human, Pair 13Observer VariationFetofetal TransfusionAbortion, LegalChorionNuchal CordTurner SyndromePregnancy, MultipleAbortion, EugenicAbortion, TherapeuticAbnormalities, MultipleBlood Flow VelocityTripletsPredictive Value of TestsReference ValuesImaging, Three-DimensionalAbortion, SpontaneousHypoplastic Left Heart SyndromeTricuspid Valve InsufficiencyRetrospective StudiesDiseases in TwinsBiological MarkersUltrasonography, Doppler, ColorFeasibility StudiesFourth VentricleColorRisk Factorsalpha-FetoproteinsUmbilical CordInfant, Newborn