The most common genetic disorder, Down syndrome affects every 1 in 800 to 1000 births. The presence of an extra chromosome 21 causes Down syndrome. The chance of having an affected child increases as a women ages, though in most cases, the mother is under the age of 35. Due to the prevalence of this syndrome, prenatal screening identifies fetuses that are at a high risk to have Down syndrome. Parents can then pursue diagnostic testing, which requires invasive procedures.
Maternal Serum Screening
The American Congress of Obstetrics & Gynecologists recommend every woman should be offered maternal serum screening. The analysis of specific patterns of proteins made by the pregnancy determines if a fetus is at increased risk for being affected with Down syndrome. Drawn in the first trimester, second trimester or both, a mother gives a sample of blood, which is analyzed in the lab. A specific risk figure is determined based on the levels of detected proteins. A positive maternal serum screen is not a diagnosis, but an indication that invasive testing may be warranted.
Nuchal Translucency
During the first trimester, a well-trained ultrasound technologist measures the thickness at the back of the fetus's neck---called the nuchal translucency. According to the American Congress of Obstetrics and Gynecologists, a larger measurement is a symptom the fetus may be affected with Down syndrome. Often, this result is combined with first-trimester maternal serum screening, creating a more accurate risk figure. A thicker nuchal translucency is also a symptom of other chromosomal disorders such as Turner syndrome.
Other Ultrasound Findings
Looking at the fetus from head to toe, a clinician can detect other symptoms of Down syndrome. The "Journal of American Medical Association" notes that alone these symptoms are not specific to Down syndrome, but may give weight to abnormal serum screen or nuchal translucency results. Detectable by ultrasound, a cardiac malformation is a common symptom. Duodenal atresia, a condition in which the first part of the small bowel has not developed properly, is another common symptom, and looks like a "double bubble" in ultrasound images. Other ultrasound symptoms that may be indicative of Down syndrome include bright spots in the bowel, heart and brain; renal abnormalities; shortened long bones; and enlarged brain ventricles.
References
- Thompson & Thompson Genetics in Metabolism (7th Edition); Robert Nessbaum; 2007
- Obstetrics & Gynecology; ACOG Practice Bulletin No. 77: Screening for Fetal Chromosomal Abnormalities
- Journal of the American Medical Association; Second-Trimester Ultrasound to Detect Fetuses With Down Syndrome
