In the past few years, the introduction of screening programs based either on maternal serum or on sono­graphic markers has aimed to reduce the number of neo­nates born with chromosomal abnormalities, especially Down’s syndrome (DS). An additional objective is to reduce the loss of fetuses from invasive procedures without compromising the detection rate of chromosomal abnor­malities.

In many countries, ultrasound scanning, using high-resolution real-time scanners, during pregnancy is a rou­tine practice in private offices and in hospitals. Over the last 15 years, there have been several studies published regarding the use of second trimester genetic sonography in the prenatal detection of fetuses with DS and other aneuploidies.

The most common chromosomal defects are trisomy 21, trisomy 18, trisomy 13, Turner syndrome (45,X), the 47,XXX, the 47,XXY, the 47,XYY, and the triploidy. In the first trimester of pregnancy, a common feature of many chromosomal defects is the increased nuchal translucency (NT) thickness. In later pregnancy, each chromosomal defect has its own syndromal pattern of abnormalities.