I am 29 years old and am 21 weeks along. I just had an ultrasound a couple of days ago and was told that the nasal bone is not showing up which puts me at higher risk for a baby with Down Syndrome. I have yet to have someone tell me how much of an increased risk. I did not have the 1st trimester screenings as I've always said that it wouldn't make any difference but now that it's staring me in the face I am seriously considering an amniocentesis. I just wonder if I can go through the next 19 weeks wondering. Can you tell me what my risk is for a Down Syndrome baby? Thank you. Posted by Tamsen to Fruit of the Womb at Wed Sep 02, 04:48:00 AM 2009
Previously we published a post that discussed the role of assessment of the fetal nasal bone in first trimester screening for fetal chromosomal abnormalities and, in particular, screening for Down syndrome (trisomy 21). Confirmed absence of the fetal nasal bone in first trimester has been correlated with a detection rate for Down syndrome in the range of 70% (with false positive rates dependent on maternal ethnicity – 2.2% in causcasians; 5% in Asians; and 9% in Afro-Carribeans) (Cicero, et al. Ultrasound Obstet Gynecol. 2003;21:15–18; Prefumo, et al., BJOG 2004; 111:109–112). Although determining the presence or absence of the nasal bone can clearly contribute to the risk assessment in first trimester, unfortunately, the technical difficulty of reliably obtaining an image and accurately interpreting the findings have led to more restricted use here in the U.S., even at many major academic centers.
In contrast, in midtrimester genetic screening, often done at 18-20 weeks, the finding of an absent nasal bone and to a lesser degree a hypoplastic nasal bone, is becoming more widely recognized as a major ‘marker’ for trisomy 21. In midtrimester, complete absence of the fetal nasal bone occurs in about one-third of Down syndrome babies. If a ‘short’ nasal bone (nasal bone hypoplasia), is included in the evaluation, 60% or more fetuses with Down syndrome may be detected, again with false-positive rates depending on ethnicity and the variable cut-off values for defining a “short nasal bone” in different studies (Bromley; et al., J Ultrasound Med 2002; 21:1387–1394; Bunduki; et al., Ultrasound Obstet Gynecol 2003; 21:156–160; Lee, et al., J Ultrasound Med 2003; 22:55–60; Gamez, et al., Ultrasound Obstet Gynecol 2004; 23:152–153).
One small study using 3D ultrasound found an absent nasal bone in 9 of 26 babies with Down syndrome (34.6%) and only 1 of 27 (3.4%) chromosomally normal babies, but this also meant that 9 of the 10 (90%) babies in whom complete absence of the nasal bone was found had Down syndrome (Goncalves, et al., J Ultrasound Med 2004;23:1619-27). In a recent study of 4373 babies evaluated in midtrimester, complete absence of the nasal bone was found in about 30% of Down syndrome and only 1% of chromosomally normal fetuses . (Odibo; et al., Am J Obstet Gynecol 2008;199:281.e1-281.e5). Nasal bone hypoplasia, defined in this study as <0.75 MoM, identified 47% of Down syndrome pregnancies and occurred in 6% of normal pregnancies.
So, to our reader, I cannot give a precise estimate of increased risk based on the ultrasound findings you report. However, if the ultrasound was performed by an experienced examiner and adequate images were obtained for evaluation, the complete absence of a fetal nasal bone at 21 weeks, even as an isolated finding, is disconcerting. The risk for Down syndrome could be as high as 90% and the false positive rate 5% or less. And, if you really need to know whether or not your baby is affected, an amniocentesis would be the best way to get that information. Best wishes and please let us know what you find out. Dr T