Identifying Ultrasound Markers for Down Syndrome.

Although the incidence of Down syndrome increases with advancing maternal age, the use of maternal age alone as a screening tool results in the identification of only about one third of the cases of fetal Down syndrome. Screening tools for Down syndrome (trisomy 21) have become more sensitive and specific during the last few years. The use of biochemical markers for the screening of patients with fetuses having chromosomal anomalies has become more widespread in the obstetric community. The triple-screen test uses maternal age plus serum alpha-fetoprotein, unconjugated estriol, and human chorionic gonadotropin levels to calculate a risk for fetal Down syndrome. Because as many as 11% to 35% of fetuses with chromosomal defects have anatomical characteristics that can be visualized on a detailed ultrasound evaluation, researchers are studying the usefulness of this imaging technique as a screening tool for Down syndrome. Ultrasound findings associated with trisomy 21 may be divided into 2 groups. The first group comprises the common major malformations associated with Down syndrome, such as duodenal atresia and cardiac disease. The second group comprises the ultrasound screening indicators, that is, anatomical malformations highly specific to Down syndrome. This group includes brachycephaly, mild ventriculomegaly, macroglossia, abnormal facies, nuchal edema, echogenic or hyperechoic bowel, pyelectasis, and shortening of the limbs. Although the diagnosis of chromosomal anomalies remains dependent on karyotyping, the use of ultrasound may limit the number of invasive procedures and allow for more accurate genetic counseling of the mother at risk for delivering an infant with Down syndrome.