Learning curve for sonographic examination of the fetal nasal bone at 11-14 weeks

To determine the number of ultrasound examinations necessary for training sonographers to examine accurately the fetal nasal bone at 11-14 weeks' gestation.

Fifteen sonographers with experience in measuring nuchal translucency were asked to examine the nasal bone during the routine 11-14-week scan. The supervising doctor recorded if the sonographer succeeded in obtaining the correct image. Each sonographer performed a total of 140 examinations, and the data were analyzed in seven groups of 20 examinations. In a second study, two sonographers with extensive experience in examining the nasal bone examined independently 100 consecutive patients at a median fetal crown-rump length of 65 (45-84) mm and median gestational age of 12 (11-14) weeks and recorded whether the nasal bone was absent or present.

In the first group of 20 examinations, there was failure to obtain the correct image of the fetal profile in 1-5 (median, 4) cases. In the subsequent three groups, there was failure to obtain the correct image in 0-3 (median, 1) cases. In the fifth and sixth groups failure occurred in 0-2 (median, 0) cases and in the seventh group all sonographers obtained successful images of the fetal profile in all cases. One sonographer obtained successful images of all cases after the first 40 scans, four after the first 60 scans, six after the first 80 and two each after the first 100 and 120 scans. In the second study, there was agreement between the two sonographers that the nasal bone was absent in two and present in 98 of the 100 consecutive patients examined.

The minimum number of scans required for an experienced sonographer to become competent in examining the fetal nasal bone is on average 80, with a range of 40-120.