Maxilla-nasion-mandible (MNM) angle: an indicator to assess fetal facial profile in first-trimester of pregnancy

Objective: The aim of this study was to observe whether there existed significant differences in the maxilla-nasion-mandible angle (MNM) between the first-and second-trimester of pregnancy, and to observe its predictive values for trisomy 18. Methods: Two experienced ultrasonologists used 2D and 3D ultrasound imaging techniques to obtain the facial sagittal sections of fetuses in the first-trimester of pregnancy (crown-rump length 45-84 mm), respectively, so as to measure MNM. Results: MNM could be measured in 91 % of normal fetuses, and the measurement differences by different operators in different groups were <1.1 degrees; average MNM was 11.0 degrees, and no significant change was observed in different gestational ages (P = 0.15). The average of MNMs in fetuses with trisomy 18 was 16.6 degrees, which were all higher than the 95th percentile of normal measurement data. The sensitivity and specificity of increased MNM on the abnormal detection of trisomy 18 were 54.7 and 97.4 %, respectively. Conclusions: The feasibility and reproducibility of measuring MNM in early pregnancy were good. MNM had certain suggestive roles for aneusomic chromosomal abnormalities, especially for fetuses with trisomy 18.