The Mid-trimester Genetic Ultrasound: Past, Present and Future

Down syndrome is the most frequent disease of mental retardation thus many tests have been developed for its prenatal screening. Among these tests, the mid-trimester genetic ultrasound examination which assesses enormous soft markers has proceeded for more than 15 years. Although there is still a debate on the efficiency of the mid-trimester genetic ultrasound, many sonographers still pay much attention to these soft makers. One of the reasons is that as maternal age increase, there are more and more pregnant women over 35 years old and they are suggested to receive an invasive mid-trimester amniocentesis. However, some of them decline to have an invasive procedure and look for a noninvasive screening. With advances in the wide acceptance of first-trimester and second trimester Down syndrome screening, pregnant women with intermediate risk hesitate in deciding whether they should take the risk of abortion and receive further amniocentesis. Many expect other non-invasive methods in evaluating their risk of Down syndrome. In this article, we review the research of all the mid-trimester soft markers including nuchal thickness, the choroid plexus, hypoplasia of the nasal bone, echogenic cardiac focus, echogenic bowel, pyelectesis and shorter femurs or humerus. We also review the history of genetic ultrasound and its efficiency. As more and more non-invasive tests for Down syndrome are established, we compare the use of the genetic ultrasound and other maternal serum marker tests, alone and in combination. The future and perspectives of mid-trimester genetic ultrasound examination are discussed. Sonography examination is still one of the most important non-invasive methods for screening Down syndrome. With more understanding of mid-trimester genetic ultrasound, we can improve the detection rate of Down syndrome and decrease unnecessary invasive procedures such as chorionic villi samplings and amniocentesises. It may also give more assurance and decrease the anxiety of high risk women older than 35 years old or those told they have an intermediate risk after receiving first trimester and second trimester Down syndrome screenings.