First-Trimester Ultrasonographic Screening for Trisomy 21 Using Fetal Nuchal Translucency and Nasal Bone

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Abstract
To report our experience with first-trimester screening for trisomy 21 by using the combination of nuchal translucency thickness and nasal bone assessment. Pregnant women from a predominantly Latin American population consisting of patients at both low risk and high risk for chromosomal defects underwent first-trimester ultrasonographic screening. Nuchal translucency thickness and nasal bone were assessed by two accredited fetal medicine specialists. Cases of trisomy 21 were identified from the cytogenetics laboratory logbook. Over a 3-year period, 1,287 consecutive singleton pregnancies were screened. The median maternal age was 33 years (range 14-47 years), with 456 (35.4%) women aged 35 years or older at the time of the scan. Overall, 110 fetuses (8.5%) had nuchal translucency thickness greater than the 95th percentile for gestational age and 25 (1.9%) had absent nasal bone. Trisomy 21 was diagnosed in 31 cases. Among them, the nuchal translucency thickness was increased in 28 and the nasal bone was absent in 13 (detection rates of 90.3% and 41.9%, respectively; P<.01). All but one (92.3%) of the trisomy 21 fetuses with absent nasal bone had increased nuchal translucency. Only two of the normal fetuses had an absent nasal bone in the first trimester. In our population, increased nuchal translucency thickness is the most important ultrasonographic marker of trisomy 21. In contrast, the nasal bone seems to have a less prominent role in identifying the fetus at risk for trisomy 21 due to its lower detection rate. However, an absent nasal bone should be considered as a highly predictive marker of trisomy 21.