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Objective To investigate the potential value of ultrasound examination of the fetal profile for present/hypoplastic fetal nasal bone at 15-22 weeks' gestation as a marker for trisomy 21. Methods This was an observational ultrasound study in 1046 singleton pregnancies undergoing amniocentesis for fetal karyotyping at 15-22 (median, 17) weeks' gestation. Immediately before amniocentesis the fetal profile was examined to determine if the nasal bone was present or hypoplastic (absent or shorter than 2.5 mm). The incidence of nasal hypoplasia in the trisomy 21 and the chromosomally normal fetuses was determined and the likelihood ratio for trisomy 21 for nasal hypoplasia was calculated. Results All fetuses were successfully examined for the presence of the nasal bone. The nasal bone was hypoplastic in 21/34 (61.8%) fetuses with trisomy 21, in 12/982 (1.2%) chromosomally normal fetuses and in 1/30 (3.3%) fetuses with other chromosomal defects. In 3/21 (14.3%) trisomy 21 fetuses with nasal hypoplasia there were no other abnormal ultrasound findings. In the chromosomally normal group hypoplastic nasal bone was found in 0.5% of Caucasians and in 8.8% of AfroCaribbeans. The likelihood ratio for trisomy 21 for hypoplastic nasal bone was 50.5 (95% C127.1-92.7) and for present nasal bone it was 0.38 (95% CI 0.24-0.56). Conclusion Nasal bone hypoplasia at the 15-22-week scan is associated with a high risk for trisomy 21 and it is a highly sensitive and specific marker for this chromosomal abnormality. [ABSTRACT FROM AUTHOR]