Fetal nose bone length: a marker for Down syndrome in the second trimester.

OBJECTIVE: To evaluate the significance of nasal bone length in relation to the detection of Down syndrome in the second trimester. METHODS: We evaluated consecutive fetuses referred to our facility between 15 and 20 weeks' gestation for sonography and amniocentesis because of an increased risk of aneuploidy. A detailed structural survey, biometric measurements, and measurement of the nasal bone were obtained at the time of amniocentesis and subsequently compared with karyotype. The characteristics of the fetuses with Down syndrome were compared with those of the euploid fetuses. RESULTS: A total of 239 fetuses were evaluated. Sixteen fetuses (7%) had Down syndrome, and 223 were euploid. In fetuses with Down syndrome, 6 (37%) of 16 did not have detectable nose bones, compared with 1 (0.5%) of 223 control fetuses, yielding a likelihood ratio of 83. Detectable nasal bones were seen in 10 fetuses with Down syndrome and 222 euploid fetuses. A receiver operating characteristic curve for the biparietal diameter-nasal bone length ratio showed that a value of 9 or greater detected 100% of fetuses with Down syndrome and 22% of euploid fetuses. If the ratio were 10 or greater, then 81 % fetuses with Down syndrome and 11 % of euploid fetuses would have been identified. If the ratio were 11 or greater, 69% of fetuses with Down syndrome would be identified, compared with 5% of euploid fetuses. CONCLUSIONS: The absence of a nasal bone is a powerful marker for Down syndrome. A short nasal bone is associated with an increased likelihood for fetal Down syndrome in a high-risk population.