Objective: To determine the range of positions of the fetal head in which a three-dimensional (3D) volume is acquired for subsequent successful assessment of the frontomaxillary facial (FMF) angle.
Method: We obtained 3D volumes of the fetal head at 11 + 0 to 13 + 6 weeks. The volumes were acquired with the head in different positions and reconstructed to obtain a mid-sagittal section and demonstrate the maxilla, palate and frontal bone, which constitute the landmarks for the assessment of the FMF angle.
Results: In the reconstructed mid-sagittal sections, it was possible to demonstrate the landmarks that define the FMF angle in most of the cases when the 3D volume acquisition plane was: (a) mid-sagittal, with the angle between the face of the transducer and the direction of the fetal nose being about 0-99 degrees, 150-199 degrees and 330-359 degrees; (b) transverse at the level of the biparietal diameter when the angle between the transducer and the midline echo of the brain was 0-29 degrees; and (c) oblique around the crown-rump axis when the angle from the mid-sagittal plane was 0-49 degrees. However, the measurement of the FMF angle was artificially increased when in the mid-sagittal plane the angle was 40-99 degrees and 150-199 degrees.
Conclusion: Successful assessment of the FMF angle by 3D ultrasound is dependent on the plane and angle of the volume acquisition.