Genetic sonogram

This ultrasound examination consists of a series of fetal images of specific tissues that help demonstrate or exclude Down syndrome. The complete genetic sonogram includes various ultrasound images of the fetus, searching to identify characteristic Down syndrome abnormalities including nuchal translucency and thickening, short femur and humerus, an absent or underdeveloped nasal bone, and sandal gap deformity of the toes. Nuchal translucency is the best and most accurate marker that is predictive of Down syndrome. The other physical characteristics, called short markers, vary in sensitivity (the ability to detect the abnormality) and accuracy in interpreting the results.

Nuchal translucency imaging is best when done in between the 11th and 14th week of gestation, as one of the recommended combined first trimester screening tests. The combined first trimester screening tests consist of nuchal translucency ultrasound imaging, human chorionic gonadotrophin (p. 193), and pregnancy-associated plasma protein A (p. 514).

For interpretation, the test results are considered as a combined result rather than as single entities. When nuchal translucency, the most sensitive of the triple markers is included, the combined first trimester screening tests have a 90% detection rate of Down syndrome and a 5% false-positive rate (Sonek & Nicolaides, 2010). The second trimester quadruple marker screening tests also assesses for indicators of Down syndrome and neural tube defects. The included tests are serum alpha-fetoprotein (p. 62), human chorionic gonadotrophin (p. 193), unconjugated estriol (p. 306), and inhibin A. The more complete genetic sonogram, including nuchal translucency, may be done late in the first trimester, or in the early part of the second trimester, in the follow-up of abnormal results of the combined first trimester screen or second trimester quadruple marker screening tests.

Nuchal translucency

Nuchal translucency or nuchal fold thickness is the most effective and accepted soft marker as an indicator of chromosomal disorder of the fetus, and Down syndrome in particular. It has higher sensitivity and a lower rate of a false-positive finding than the other soft markers. There is a 4.7% false-positive rate with the nuchal translucency measurement. A false-positive result means that the tests indicate abnormality, but in reality, the fetus is normal.

The nuchal tissue is located in the back of the fetal neck. Nuchal fold translucency is measurable by ultrasound in the 11th to 14th week of gestation; the tissue appears translucent because of fluid or edema. On the sonogram image, the thickness of the translucent tissue of the nuchal fold is measured from the outer surface of the occipital bone to the exterior of the fetal scalp (Figure 52). When the measurement is greater than the reference value, it is an indicator of risk for Down syndrome, cardiac anomalies, and other fetal defects (Sonek and Nicolaides, 2010).

Figure 52. Nuchal translucency. Abnormally large nuchal translucency measurement (arrows) of 4.1 mm. This fetus had trisomy 21.

(From Rumack C, Wilson SR, Charboneau JW: Diagnostic ultrasound, ed 3, St Louis, 2005, Mosby.)

Carrier screening for cystic fibrosis

To identify the carriers of this genetic mutation, genetic screening is offered to the female before conception or at the time of the first prenatal visit to her doctor. Because in the United States it is increasingly difficult to identify a single race or ethnic background, the current recommendation is to offer CF testing to all women, regardless of ethnic or racial status (Goetzinger & Cahill, 2010).

If the woman tests positive for this mutation, she is given the information and genetic counseling. Then, her reproductive partner is offered testing. Alternatively, both partners may be tested simultaneously. If the partner also tests positive, both are given genetic counseling. They receive a thorough explanation of cystic fibrosis and their risk of transmitting the disease or the genetic carrier status to their offspring. They are also counseled that DNA testing does not always identify the genetic mutations when they actually exist.

Fetal screening for cystic fibrosis

If the fetus is to be tested genetically for the disease, chorionic villus sampling is done at 10 to 13 weeks’ gestation or amniocentesis is done at 15 to 20 weeks’ gestation. If the fetus tests positive for the homozygous genetic mutation of cystic fibrosis disease, the couple is given the information and counseling by the genetics counselor and their physician. They have a choice to terminate the pregnancy or carry the baby to term and prepare for the delivery of a child who will have many health care needs.