The thickness of the tissue posterior to the cervical spine at 10–14 weeks, called the nuchal translucency, has been found to have important prognostic significance. Fetuses with abnormal thickening, which is likely due to soft-tissue edema related to a lymphatic abnormality, have an increased likelihood of chromosomal abnormalities, cardiac anomalies, various syndromes, and spontaneous miscarriage. Specific chromosomal anomalies associated with nuchal translucency thickening include 45, X (Turner syndrome) and trisomy 21 (Down syndrome), as well as trisomy 13 and trisomy 18.

The nuchal translucency measurement can be used in conjunction with tests of the maternal serum for human chorionic gonadotropin (hCG) and pregnancy-associated plasma protein A (PAPP-A) to assess the likelihood of fetal aneuploidy. This combined test is commonly offered to patients as a screening test for aneuploidy in the late first trimester.

If the nuchal translucency is greater than 3 mm in an 11–14-week fetus, whether or not the combined test was performed, the parents are typically offered genetic counseling. Even if the karyotype proves to be normal (or the parents elect not to undergo karyotype testing), a repeat sonogram at approximately 18 weeks is usually recommended to assess for cardiac or other structural anomalies.

The nuchal translucency in an 11–14-week fetus appears as a sonolucent region between the spine and posterior skin (Figure 4.1.1). The measurement is obtained from a midline sagittal view of the fetus with the fetus in neutral position, neither hyperflexed nor hyperextended (Figure 4.1.2) at the neck, such that some amniotic fluid is visible between the chin and the chest, and the angle of the chin to the chest is not more than 90 degrees. The ultrasound view should be magnified or enlarged such that the fetus takes up more than half the image. Care must be taken to distinguish the posterior skin surface from the amnion (Figure 4.1.3). Abnormal thickening of the nuchal translucency (Figures 4.1.3 to 4.1.5) is associated with an increased risk of aneuploidy, heart defects, and many congenital and genetic abnormalities. The greater the thickening, the higher the risk of aneuploidy and other defects. If aneuploidy is excluded by genetic testing and the detailed 18–20-week ultrasound and echocardiogram are normal, the chances that the newborn will be normal are excellent.

Extremely thick nuchal translucencies, those greater than 5 mm, are often septated and called cystic hygromas (Figure 4.1.6), although these findings are considered part of the spectrum of nuchal thickening. With this extensive lymphatic abnormality, cystic hygromas can extend along the back and front of the fetus (Figures 4.1.7 and 4.1.8), a finding with a particularly high risk for aneuploidy.

During the latter weeks of the first trimester, from approximately 10 weeks onward, many fetal organs and structures are visible. As a result, a number of fetal anomalies can be detected at sonography between 10 and 14 weeks gestation. In particular, anomalies that involve the outer contour of the fetus, such as anencephaly, anterior abdominal wall defects, amniotic bands, and missing limbs, can be diagnosed in many cases. Anomalies of the internal organs, such as the brain, kidneys, and heart, are more difficult to detect because of their small size during this period; however, in some cases, anomalies of internal organs can be detected. In general, the more severe the anomaly, the more likely it is that the abnormality can be diagnosed by sonography between 10 and 14 weeks.