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°; average MNM was 11.0°, 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°, 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. [ABSTRACT FROM AUTHOR]