To assess the intra- and interobserver reproducibility in the measurement of the frontomaxillary facial (FMF) angle at 11 + 0 to 13 + 6 weeks' gestation and to investigate the effect of deviations from the exact mid-sagittal view on these measurements.
Three-dimensional (3D) volumes of the fetal face were used by two operators to measure the FMF angle in 50 chromosomally normal and 50 trisomy 21 fetuses. The measurements were taken in the exact mid-sagittal view and repeated after lateral rotation of the head by 5°, 10° and 15° away from the vertical position of the occipitofrontal diameter axis. Mean difference and 95% limits of agreement between paired measurements of FMF angle by the same and by two different sonographers were determined.
In the mid-sagittal plane the maxillary bone was rectangularly shaped. Rotation away from this plane became easily recognizable because at a mean of 7° (range, 4–10°) the shape of the maxilla changed with the appearance of the zygomatic process of the maxilla and at a mean of 8° (range, 4–12°) the tip of the nose became invisible. In both the normal and trisomy 21 fetuses the FMF angle measured at 5–15° was not significantly different from the one measured in the mid-sagittal plane. In 95% of the cases, the difference between paired measurements of the FMF angle by the same sonographer at the mid-sagittal plane was between − 2.3° and 3.0° and at 15° it was − 1.0° to 6.8°. At the mid-sagittal plane, the difference in measurements between two sonographers was − 3.1 to 3.0°.
The landmarks that define the mid-sagittal plane of the fetal face are the tip of the nose and the rectangularly shaped maxilla. Measurement of the FMF angle is highly reproducible. Copyright © 2006 ISUOG. Published by John Wiley & Sons, Ltd.