Absence of nasal bone in fetuses with trisomy 21 at 11-14 weeks of gestation: an observational study.

BACKGROUND: Prenatal diagnosis of trisomy 21 requires an invasive test in women regarded as being at high risk after screening. At present there are four screening tests, and for a 5% false-positive rate, the sensitivities are about 30% for maternal age alone, 60-70% for maternal age and second-trimester maternal serum biochemical testing, 75% for maternal age and first-trimester fetal nuchal translucency scanning, and 85% for maternal age with fetal nuchal translucency and maternal serum biochemistry at 11-14 weeks. In this study, we examined the possible improvement in screening for trisomy 21 by examining the fetal nasal bone with ultrasound at 11-14 weeks of gestation.

METHODS: We did an ultrasound examination of the fetal profile in 701 fetuses at 11-14 weeks' gestation immediately before karyotyping for a possible chromosomal abnormality detected by maternal age and fetal nuchal translucency screening. The presence or absence of a nasal bone was noted.

FINDINGS: The fetal profile was successfully examined in all cases. The nasal bone was absent in 43 of 59 (73%) trisomy 21 fetuses and in three of 603 (0.5%) chromosomally normal fetuses. The likelihood ratio for trisomy 21 was 146 (95% CI 50-434) for absent nasal bone and 0.27 (0.18-0.40) for present nasal bone. In screening for trisomy 21, by a combination of maternal age and fetal nuchal translucency, we estimated that inclusion of examination of the fetal profile for the presence or absence of nasal bone could increase the sensitivity to 85% and decrease the false-positive rate to about 1%.

INTERPRETATION: In screening for trisomy 21, examination of the fetal nasal bone could result in major reduction in the need for invasive testing and a substantial increase in sensitivity.