Objectives: To investigate the utility of FMF angle screening in routine assessment at 11-14 weeks gestation. To determine if the accuracy of 2D (real-time) FMF angle measurement is comparable to that obtained by 3D (off-line) volume reconstruction.
Methods: A single operator collected 2D images and 3D volumes from single chromosomally normal fetuses at 11-14 weeks gestation in consenting participants. The FMF angle was firstly measured on a frozen 2D image in an appropriate plane and, after a delay of at least 48 hours, was determined on the reconstructed 3D image. Half of the 3D volumes were again reviewed by the same operator after at least 7 days.
Results: Successful visualization of the FMF angle was possible in 79 / 87 (91%). The range of FMF angles was 78.5 – 95.6° (2D) and 78.8 – 95.3° (3D). The FMF angle reduces with advancing CRL. In the volumes with paired measurements, the intra-observer difference was less than 5° in 90% of cases. The difference between the paired 3D and 2D measurements overall was less than 8° in 90% of cases. This difference reduced substantially after an initial learning curve of 40 cases.
Conclusions: FMF angle measurement is repeatable and can be incorporated into routine 11-14 week screening without lengthening the examination. Fetal position precludes accurate measurement in a significant percentage. With appropriate training and experience, 2D measurement of the FMF angle is just as accurate as 3D volume reconstruction.