Prenatal screening for fetal aneuploidies during the first and second trimester of pregnancy

Prenatal screening for chromosomal defects during the first and second trimesters of pregnancy has become an established part of obstetric practice in many countries. The goal of current maternal serum screening programmes is to identify women who are at an increased risk of having a baby affected with Down syndrome (trisomy 21), Edwards syndrome (trisomy 18) or neural tube defects and who will benefit from such diagnostic tests. The most commonly used test for genetic diagnosis is amniocentesis; however, the rate of spontaneous fetal loss caused by this test averages at about 1 in every 200 procedures. Because of this risk, serum analyte testing has become an important and non-invasive first step in detecting patients at risk of carrying a child with congenital abnormalities. The non-invasive screening options which are currently available to patients include combining maternal age with one of the following: first-trimester serum screening [nuchal translucency (NT) and maternal serum biochemistry markers]; second-trimester serum screening (maternal serum biochemistry markers such as the triple test and the quadruple test); or the two-step integrated screening, which comprises first- and second-trimester serum screening with or without NT.