Nasal bone in prenatal trisomy 21 screening

The definitive diagnosis of fetal chromosomal abnormalities is only accomplished through tests that sample fetal tissue. These procedures--amniocentesis, chorionic villus sampling and cordocentesis--are invasive and carry with them the risks of bleeding, rupture of membranes, and even pregnancy loss. Current recommendations from the American College of Obstetricians and Gynecologists state that all women should be offered screening, regardless of maternal age. However, the exact screening test to apply to pregnant women is still a matter of debate. The goal of any screening method should be high detection (sensitivity) at low screen positive rates. Recent attention has focused on additional ultrasound markers to potentially improve the detection rate. Given that mid-face hypoplasia and a flattened nose are characteristic features of Down syndrome, investigators have tried to utilize its prenatal appearance--or more specifically, its absence--to enhance the detection of trisomy 21 (T21). The purpose of this document is to review the data on the utility of the nasal bone as a marker for T21. Particular attention will focus on its use in the first trimester versus second trimester as well as future directions for its potential incorporation into screening strategies.

Obstetricians & Gynecologists, Family Physicians.

After completion of this article, the reader will be able to relate the prenatal and postnatal nasal phenotypes for Down syndrome, identify the potential role and pitfalls of nasal bone characteristics in screening for Down syndrome, and describe 3 different methods to define fetal nasal bone hypoplasia.