Prenasal thickness in trisomy-21 fetuses at 16-24 weeks of gestation.

OBJECTIVE To construct a reference range for fetal prenasal thickness between 16 and 24 weeks of gestation and to evaluate the thickness in fetuses with trisomy 21. METHODS We acquired three-dimensional (3D) volumes of the fetal profile from 135 normal fetuses and 26 fetuses with trisomy 21 at 16-24 weeks' gestation. We used the multiplanar mode to obtain the exact mid-sagittal plane and measured the prenasal thickness as the shortest distance between the anterior edge of the lowest part of the frontal bone (at the junction with the nasal bone when present) and the skin anteriorly. RESULTS In the normal group prenasal thickness increased with gestation from a mean of 2.4 mm at 16 weeks to 4.6 mm at 24 weeks. Repeatability studies demonstrated that in 95% of the cases the difference between two measurements of prenasal thickness by the same operator and by different operators was less than 1 mm. In the trisomy-21 fetuses the mean prenasal thickness was significantly larger than in normal fetuses and in 19 (73.1%) cases it was above the 95(th) centile of the normal range. There was no significant difference in prenasal thickness between the trisomic fetuses with and without ventriculomegaly, nuchal edema, absent nasal bone or a cardiac defect. CONCLUSIONS The fetal profile is routinely examined during the second-trimester scan and therefore the skill needed to obtain the view necessary for the measurement of prenasal thickness is widely available. If the finding of our study--that in more than 70% of fetuses with trisomy 21 prenasal thickness is above the 95(th) centile--is confirmed in prospective screening studies this measurement alone could prove a highly sensitive method of second-trimester screening for trisomy 21.