The second-trimester fetus with Down syndrome: detection using sonographic features.

Down syndrome (trisomy 21) is the most common chromosomal abnormality among liveborn infants in the United States, with an incidence of 1/700 to 1/1000 births. Its incidence rises with maternal age, thus prompting the current practice of offering genetic amniocentesis to women 35 years of age or older. However, even if every pregnant woman of advanced maternal age (> or = 35 years) were to undergo karyotyping, only 20% of fetuses with Down syndrome would be detected. Until approximately 10 years ago, karyotyping of the fetuses of older women was practically the only antenatal method of identifying any fetus with trisomy 21. In 1984, Cuckle and associates first reported the use of a low level of serum alpha-fetoprotein as a screening tool for identifying affected fetuses in younger women. Combining the criteria based on both maternal age and serum alpha-fetoprotein resulted in identifying 40% of fetuses with Down syndrome, as well as 6.8% of unaffected fetuses. Still, at least 60% of fetuses with Down syndrome remained undetected with these screening techniques. It is logical to expect that sonographic imaging of the second-trimester fetus itself would result in the optimal detection of the anatomical features of Down syndrome (Figure 1). To this end, many researchers have attempted to identify sonographic findings that would serve to detect fetuses likely to have Down syndrome. This review summarizes the characteristic sonographic findings associated with an increased risk of a fetus having trisomy 21.