Ultrasound Markers of Chromosomal Anomalies in the First Trimester

Chromosomal anomalies are associated with considerable morbidity and mortality. The protocol for identifying these fetuses had for many years included a single clinical criterion of maternal age. Advances in biochemical screening combined with the excellent display of fetal dysmorphology afforded by technological advances in ultrasound equipment have resulted in a paradigm shift in the diagnosis of chromosomal abnormalities in the fetus, from the second trimester to the late first trimester. The accuracy of diagnosis as reported in multiple large series has pushed both screening and diagnostic testing for chromosomal disorders to the window now referred to as the 11 to 13 weeks + 6 days scan. Recent data have shown chorion villus sampling after 10 weeks to be as safe in experienced hands as amniocentesis and this has pushed the advantages of first trimester screening further. Ultrasound parameters for the detection of Down’s syndrome in the first trimester include the nuchal translucency (NT) as the most well-defined and studied parameter, evaluation of the nasal bone (NB), frontomaxillary facial (FMF) angle, ductus venosus (DV) flow velocity waveform, tricuspid regurgitation (TR) and fetal heart-rate. Each parameter has well-defined criteria to be fulfilled for accurate quantification. Biochemical parameters that are currently in wide use include PAPP-A and free beta-hCG. Other parameters that the software accounts for are the gestational age assessed by the crown-rump length, maternal age, ethnicity, smoking, IVF and number of fetuses with chorionicity. Combining maternal age, biochemistry, NT and NB between 11 to 13 weeks + 6 days yields a detection rate of 96% with a false positive rate of 5%.