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MS WordPerinatologist Corner - C.E.U/C.M.E. Modules
Prenatal Genetic Screening – Serum and Ultrasound
Sponsored by The Indian Health Service Clinical Support Center
8. Ultrasound markers of fetal Down Syndrome
What about ultrasound markers of fetal DS ?
The so-called second trimester “genetic sonogram”?
Ten to 40% of DS fetuses may have some anatomic abnormality, such as a major cardiac anomaly, which can be identified antenatally and raise the risk of aneuploidy significantly. The thickened nuchal fold (>5 mm at 15-21 weeks) is the most reliable “soft marker” of fetal DS, raising the likelihood ratio significantly. Other more subtle findings such as hyperechoic bowel, pyelectasis, shortened femur or humerus, echogenic intraventricular foci, clinodactyly, “sandle-gap toes”, or choroid plexus cysts may also be present. The positive predictive value of any of these ultrasound indicators in isolation is very low, but the presence of two or more positive findings, especially in the presence of an abnormal multiple marker screen, may increase the risk enough that many women would more strongly consider amniocentesis. On the other hand, a negative scan, with no discernible anomalies or soft markers, lowers the risk of aneuploidy by approximately 40%, and is very reassuring to many women.
How about the other less common trisomies?
MMS will also detect about 80% of trisomy-18 fetuses (all analytes low), as well as Turner syndrome and triploidy, but not trisomy-13 or translocations. Ultrasonographic anatomic abnormalities may also be much more striking in such fetuses. Twins may have a higher risk of one of the fetuses in the pregnancy having an abnormal karyotype. Since there are two fetuses, they may be discordant for a chromosomal abnormality, and the risk for the entire pregnancy is doubled. The presence of multiple gestation is important information for the laboratory to have when calculating the risks for both DS and NTD as the levels of the analytes will be accordingly increased.
