Ultrasound evaluation of inferior facial angle (IFA) in a population of normal fetuses at 11-13 weeks (+6 days) gestation.

Retrognathia is a common feature of many fetal anomalies that may be diagnosed during ultrasound examination. The most common method used for the diagnosis of this condition is the inferior facial angle (IFA). The existing reference ranges for IFA have been established for the fetuses at 18- 28 weeks gestation and may be not applicable in the first trimester. The aim of the study was to document changes of IFA from 11+0 to 13+6 weeks gestation, to verify the reproducibility of IFA calculations obtained by the same examiner and to investigate the possible association between IFA values at the first and second trimester scans.

Material and methods: A prospective study was performed in 512 singleton pregnancies during the first trimester ultrasound exam. IFA was calculated by measuring the angle made by the cross-section of a line orthogonal to the forehead at the level of nasofrontal suture and the line from the tip of the mentum to the anterior border of the more protrusive lip. In 100 cases, stored images were used to calculate IFA twice by the same examiner. In a second trimester scan, IFA was measured again in 215 fetuses.

Results: The median IFA value at the time of the first trimester (IFA-1) was 880 (IQR: 8.0) and decreased with crown-rump length (CRL) and biparietal diameter (BPD). A moderate negative correlation was shown between IFA-1 and CRL (r= 0.3; p<0.001) as well as between IFA-1 and BPD-1 (r= 0.176; p=0.010). An intraclass correlation coefficient (ICC) of 0.87 (95% CI: 0.81 – 0.91) confirmed strong intraobserver agreement between two IFA measurements. In 215 fetuses that underwent a second trimester screening, the median IFA-2 was significantly lower compared to IFA-1(215) (750; IQR: 10.0 vs.870; IQR: 8.0; p<0.001), and there was no correlation between IFA1215 and IFA-2 (r= -0.024; p=0.731).

Conclusions: In the first trimester, IFA decreases with CRL and BPD. When performed by the same operator, measurement of IFA is reproducible. There is no correlation between the IFA values in the first and second trimesters.