Introduction & Background: Screening for Down syndrome is one of the oldest and most commonly used is-sues of prenatal screening. The aim of the screening is to select those cases who are at high risk for Down syn-drome and justify an invasive prenatal diagnostic procedure on them. The most significant advancement in first trimester ultrasound screening has been the recognition of nuchal translucency (NT) as a potent marker for fetal aneuploidy. NT test is the most effective prenatal screening test to identify those at risk for trisomy 21. If NT levels increase above the 95th percentile (up to 3.4mm), the chance for a healthy baby will be decreased remarkably by combining sonographic findings and measurements of biochemical markers (Free BHCG, PAPP-A). An improvement in prenatal screening for chromosomal defects has also been achieved in recent years. These techniques altogether provide a detection rate of 90 % for the most important chromosomal anomaly (i.e Down syndrome) with the false positive rate of 5%. Meamwhile in cases of trisomy 21, nasal bone is not visible at the end of first trimester. Combining the delay in nasal bone formation with other first trimes-ter screenings, detection rate of trisomy 21 may increase to 97% and the false positive rate decline to 3%.