Clinical meaning
Advanced prenatal assessment integrates maternal-fetal physiology, genetic screening science, and fetal surveillance techniques to identify and manage high-risk conditions before they become emergencies. The physiological basis of prenatal screening relies on detecting biomarkers that reflect fetoplacental function. Cell-free fetal DNA (cfDNA) in maternal blood originates from placental trophoblast apoptosis: as syncytiotrophoblast cells are shed into maternal circulation, they release DNA fragments that are fetal in origin (actually placental, which is why confined placental mosaicism can cause false-positive results). By 10 weeks gestation, fetal cfDNA constitutes 10-20% of total cell-free DNA in maternal plasma, enabling detection of fetal aneuploidy through shotgun sequencing or targeted analysis — overrepresentation of chromosome 21 sequences indicates trisomy 21 with >99% sensitivity. First-trimester nuchal translucency (NT) screening measures fluid accumulation behind the fetal neck at 11-14 weeks: increased NT (>3.0 mm) occurs because lymphatic drainage is developmentally delayed in aneuploid fetuses and because cardiac defects cause jugular venous congestion — elevated NT is associated with trisomies 21, 18, 13, Turner syndrome, and congenital heart defects. Fetal surveillance in the third trimester assesses placental function and fetal well-being: the non-stress test (NST) evaluates fetal heart rate accelerations (a reactive NST shows ≥2 accelerations of ≥15 bpm lasting ≥15 seconds within 20 minutes), which reflect intact fetal autonomic nervous system function — loss of reactivity may indicate fetal hypoxia, acidosis, sleep cycle, or medication effect (betamethasone temporarily reduces variability for 24-48 hours). The biophysical profile (BPP) combines five parameters (fetal breathing movements, gross body movements, fetal tone, amniotic fluid volume, and NST), each scored 0 or 2, reflecting fetal CNS oxygenation — these parameters are lost in a predictable sequence as hypoxia worsens (heart rate reactivity lost first, then breathing movements, then movement, then tone). Umbilical artery Doppler velocimetry assesses placental vascular resistance: absent end-diastolic flow (AEDF) indicates 60-70% of placental vascular bed is obliterated, while reversed end-diastolic flow (REDF) is a pre-terminal finding with >50% perinatal mortality, indicating the fetal heart cannot generate sufficient pressure to maintain forward flow through the severely diseased placenta throughout diastole.