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Fruit of the Womb
Fruit of the Womb

Fetal Nasal Bone Assessment in First Trimester Screening for Aneuploidy

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The reader below underwent combined first trimester screening for aneuploidy and during the process, the baby was found to have an absent nasal bone. Due to the difficulty in routinely obtaining a definitive assessment of the nasal bone, and its current limited usefulness in screening women of all ages and ethnicities, many providers have elected not to include that in their certification for routine first trimester screening...

• At Fri Jun 27, 12:34:00 PM 2008, Anonymous said…

Hello,
I'm 12 weeks and 6 days and had first trimester screening done two days ago… I'll be 38 years old when this baby is born. The results were very bad due to the lack of nasal bone... my risk factor is 1/20 for Down syndrome. I have read all through your blogs and don't see anyone else with this problem... I'm very scared now and wish I would have not had the test. How common is it for a nasal bone to develop slowly? I have two other healthy children. The neck fluid (nuchal translucency) was fine though. Any information you can provide would be very much appreciated... I'm very panicked. Thank you.


• At Thu Jul 03, 07:59:00 PM 2008, Kenneth F. Trofatter, Jr., MD, PhD said…

To anonymous June 27: Sorry, I missed your comment before today. Not everyone who does first trimester screening (FTS) is "certified" to perform nasal bone (NB) assessment and even among those who are, it has been found to be difficult to perform consistently (Rosen, et al., Obstet Gynecol 2007;110:399-404). At this point, the greatest value appears to be not necessarily in including it in the screening of all women for aneuploidy in first trimester, because of the low positive predictive value in unselected populations, but as an adjunct to screening “high risk” populations such as women who will be 35 years or older at delivery and those who carry balanced translocations involving chromosome 21 (Prefumo, et al., Am J Obstet Gynecol 2006;194:828-33).

Publications by Cicero and colleagues in 2001 (Lancet 2001;358:1665-7) and 2003 (Prenat Diagn 2003;23:306-10) reported finding the absence of nasal bones in about 2/3 of first trimester screenings of babies with Down syndrome (trisomy 21) compared to 1% or less of chromosomally normal babies. By including nasal bone screening in their FTS, they were able to detect more than 90% of babies with a false positive rate of only 0.5% (and 97% at a false positive rate of 5%). As confirmed in the recent study whose abstract is below. including the nasal bone in the first trimester screen may not increase the detection rate of Down syndrome, but it does help reduce the "false positive rate" as had previously been demonstrated by Cicero and others. In other words, if the nasal bone is present, the baby is less likely to have a "positive screen" under circumstances in which the baby really does not have Down syndrome.

By the way, you stated your results “were very bad due to the lack of nasal bone." But I am curious to know what were the results of the other components of your first trimester screen. Best regards and please let us know what you find out.
Dr T

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Has, et al., Fetal Nasal Bone Assessment in First Trimester Down Syndrome Screening. Fetal Diagn Ther 2008;24:61-66.
Objective: To evaluate the contribution of nasal bone assessment in the first trimester Down syndrome screening. Methods: The fetuses which underwent first trimester screening with nuchal translucency (NT) measurement were evaluated for the absence or presence of nasal bone according to the instructions described by the Fetal Medicine Foundation, London. Results: Among the 1,807 fetuses included in the study, 9 had trisomy 21. The detection rate of Down syndrome with NT measurement was 77.8% (7/9) with a false-positive rate of 4.5%. Incorporation of biochemical tests (PAPP-A, and free beta-hCG measurement) into the screening increased the detection rate to 88.9% (8/9) and decreased the false-positive rate to 3.6%. The prevalence of absent nasal bone was 7/1,798 (0.39%) in chromosomally normal fetuses, and 3/9 (33.3%) in Down syndrome fetuses. Sensitivity, specificity, positive predictive and negative predictive values of absence of nasal bone for trisomy 21 are 33.3% (CI: 0.12-0.64), 99.6% (CI: 0.99-0.99), 30% (95% CI: 0.11-0.53) and 99.7% (95% CI: 0.99-0.99), respectively. The positive likelihood ratio of absent nasal bone was 85.6 (95% CI: 26.2-279.5), and the negative likelihood was 0.67 (95% CI: 0.42-1.06). When nasal bone assessment was incorporated into the NT risk assessment or combined test, the detection rate of trisomy 21 was not changed, however, the false-positive rate decreased to 3.4 and 3%, respectively. Conclusion: The absence of fetal nasal bone has a high positive likelihood ratio for Down syndrome in the first trimester screening, and the presence of nasal bone may potentially lower the need for invasive testing. Copyright © 2008 S. Karger AG, Basel.
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