Reference Values for the Fetal Maxilla–Nasion–Mandible ( MNM ) Angle Measured by Two‐Dimensional Ultrasound at 11 + 0 to 13 + 6 Weeks of Gestation

BACKGROUND: First-trimester ultrasound is a key component of the early assessment of fetal craniofacial anatomy. The fetal maxilla-nasion-mandible (MNM) angle has been proposed as an objective parameter for evaluating the fetal facial profile. However, reference values for this measurement during the first trimester remain limited, particularly across different populations. OBJECTIVE: To establish reference values for the fetal maxilla-nasion-mandible (MNM) angle in structurally normal singleton fetuses undergoing first-trimester ultrasound screening between 11 + 0 and 13 + 6 weeks of gestation. METHODS: This retrospective cross-sectional study included consecutive singleton pregnancies with structurally normal fetuses undergoing routine first-trimester ultrasound screening between 11 + 0 and 13 + 6 weeks of gestation. The MNM angle was measured in a standardized midsagittal plane using two-dimensional ultrasound. Fetuses with a crown-rump length of 45-84 mm and normal prenatal and postnatal outcomes were included in the reference population. RESULTS: A total of 273 fetuses were included. The mean MNM angle was 11°. No correlation was found between the MNM angle and biparietal diameter (p = 0.263), abdominal circumference (p = 0.0700), or gestational age (p = 0.533). Conversely, significant correlations were observed with head circumference (p < 0.001), femur length (p = 0.001), and the frontomaxillary facial angle (FMFA) (p = 0.001). CONCLUSION: Measurement of the fetal MNM angle by two-dimensional ultrasound between 11 + 0 and 13 + 6 weeks of gestation is feasible. The reference values established in this study provide a foundation for future prospective studies evaluating the potential clinical applicability of this parameter in fetuses with craniofacial abnormalities.