[Ultrasonographic fetal nasal bone assessment in prenatal screening for Down syndrome]

To investigate the clinic value of ultrasonographic fetal nasal bone examination as a screening marker for Down syndrome (DS).

The study was conducted in the First Affiliated Hospital of Sun Yat-sen University from Oct 2004 to Mar 2007. Two-dimensional ultrasound was used to assess the fetal nasal bone of 1863 normal pregnancies (normal group) and 25 cases with DS fetus (study group) during their second and third trimesters. The incidence of nasal bone absence or short nasal bone in two groups was determined. The fetal nasal bone absence should be confirmed in three orthogonal planes of the fetal face, and the short nasal bone included the cases that the fetal nasal bone was shorter than the 2.5th percentile of normal according to the gestational week. The diagnostic test index was used for assessing the value of fetal nasal bone abnormality as a marker in prenatal screening for DS.

(1) 1761 fetuses of normal group were successfully examined for the nasal bone and the detection rate was 94.5% (1761/1863). 102 fetuses failed examination because of inconvenient intra-uterine position. (2) The nasal bone length grew in a linear fashion throughout pregnancy and the growth pattern correlated well with gestational age (r = 0.605, P < 0.05) in normal group. The nasal bone was absent in 3 normal fetuses (0.2%, 3/1761) and short nasal bone was found in 44 normal fetuses (2.5%, 44/1761). (3) The nasal bone was absent in 7 DS fetuses (28.0%, 7/25) and short nasal bone was found in 15 DS fetuses (60.0%, 15/25). (4) When the absence of nasal bone was used as a cut-off, the sensitivity for DS was 28.0%, the specificity was 99.8%, the positive likelihood ratio was 164.45 (95% CI: 45.11-599.60), and the negative likelihood ratio was 0.72 (95% CI: 0.57-0.92). When short nasal bone was used as a cut-off, the sensitivity was 60.0%, specificity was 97.5%, the positive likelihood ratio was 24.03 (95% CI: 7.15-80.71), and the negative likelihood ratio was 0.41 (95% CI: 0.29-0.59).

Fetal nasal bone hypoplasia at the second and third trimester scan is associated with a high risk for Down syndrome and it can be used as a screen marker for this chromosomal abnormality.