First-trimester screening for Down syndrome, a genetic disorder caused by the presence of an extra chromosome, usually involves a combination of maternal blood tests and an ultrasound exam (ie, a combined first-trimester screening [FTS]), which is performed between 11 and 14 weeks of pregnancy.
During the ultrasound exam, the healthcare provider evaluates various markers that can indicate an increased risk of Down syndrome, such as the thickness of the nuchal translucency (a fluid-filled space at the back of the fetus’s neck) and the presence of certain physical features, including the nasal bone. Because research has shown that fetuses with Down syndrome are less likely to have a visible nasal bone on ultrasound than fetuses without the condition, evaluating the nasal bone can help healthcare providers assess the risk of Down syndrome more accurately.
Although visualizing the nasal bone is not a mandatory component of the screening, the inclusion of fetal nasal bone evaluation in the screening improves the clinical performance of the screen for the detection of fetal Down syndrome. Unfortunately, factors such as maternal body habitus (such as increased body mass index [BMI]), poor acoustic windows, unfavorable fetal position, delayed nasal bone ossification, and early gestational age can hinder nasal bone visualization then. In addition, ethnicity may also affect the visualization of the fetal nasal bone as the development of the fetal nasal bone differs between populations. Acknowledging and integrating the differences in facial structure between different racial and ethnic groups can help to promote equity in prenatal imaging and ensure the provision of accurate, personalized risk counseling across patient populations.
Therefore, a recent study aimed to determine if repeat nasal bone evaluation provided a significant improvement in refining the specificity of Down syndrome risk assessment by combined FTS, as well as determine the efficacy of a repeat nasal bone evaluation across various maternal ethnicities.
The study reviewed the medical records of patients who underwent a first-trimester ultrasound evaluation in an American Institute of Ultrasound in Medicine (AIUM)-accredited center between January 2015 and January 2018. The study focused on patients with fetal nasal bone labeled as “absent or hypoplastic” or “unable to be adequately visualized” during the ultrasound. The researchers reviewed the records to assess factors such as patient age, ethnicity, follow-up evaluations, and fetal anomalies. They analyzed the combined FTS results and followed up with patients with abnormal results. They then conducted statistical analyses to compare patient ethnicity and nasal bone visualization on the second exam, as well as to compare patient ethnicity and fetal Down syndrome risk by combined FTS.
The study identified 589 cases (8.7%) of absent or uncertain fetal nasal bone on initial nuchal translucency (NT) ultrasound evaluation among the 6780 total NT ultrasounds performed, with the most frequently represented ethnicities being African American/Caribbean (46.2%) and White (36.8%). Of the total, 125 patients (21.2%) did not complete a repeat nasal bone evaluation, and 105 patients with additional risk factors pursued genetic counseling. Of these patients, 20 pursued chorionic villus sampling (CVS), and 11 of these cases (55.0%) reported abnormal karyotypes. Of the 376 eligible patients who completed a second nasal bone evaluation (exam 2), 82 patients (21.8%) had an absent fetal nasal bone, 26 (6.9%) had an uncertain fetal nasal bone, and 268 (71.2%) had a present fetal nasal bone. White patients were statistically significantly more likely than African American/Caribbean patients to have a present nasal bone on exam 2 (82.9% and 59.2% respectively, P < .0001), as were Asian Indian patients (100% and 59.2%, respectively, P < .0001).
Combined FTS can identify fetuses at high risk for aneuploidy, but it has a relatively high false positive rate. Therefore, proper identification of the absence or presence of the fetal nasal bone during FTS plays a critical role in identifying and counseling patients at increased risk for fetal aneuploidy. The study found that repeat nasal bone evaluation could reduce false positive FTS results, particularly in African American/Caribbean populations.
Even with the widespread use of cell-free DNA screening, combined FTS, including nasal bone assessment, remains an important tool for first-trimester aneuploidy risk assessment.
To read more about this study, download the Journal of Ultrasound in Medicine article, “Follow Your Nose: Repeat Nasal Bone Evaluation in First-Trimester Screening for Down Syndrome” by Kristen A. Miller, MGC, et al. Members of the American Institute of Ultrasound in Medicine (AIUM) can access it for free. Join the AIUM today!
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