Antenatal ultrasound diagnosis of fetal micrognathia: validation and reproducibility of quantitative methods.

OBJECTIVES: The antenatal ultrasound diagnosis of fetal micrognathia can be made subjectively or objectively. We aimed to validate quantitative ultrasound methods proposed for the diagnosis of fetal micrognathia in the second and third trimesters of pregnancy by evaluating their diagnostic performance, reproducibility and impact of measurement variability on diagnosis. METHODS: This was a retrospective study analyzing the objective diagnostic accuracy, reproducibility and impact of measurement variability of mandible angle measurements from facial profile images of fetuses with suspected micrognathia diagnosed antenatally using a subjective method. Fetuses were recruited at between 18 and 28 weeks' gestation, at University College London Hospital, UK, between 2000 and 2018. Four quantitative methods were compared: measurement of the inferior facial angle (IFA), the fronto-naso-mental angle (FNMA), the maxilla-nasion-mandible angle (MNMA) and the facial maxillary angle (FMA). Methods were validated by calculating the percentage of cases in which the measured angles met the diagnostic criteria for micrognathia. To assess intra- and interobserver reproducibility, two sonographers, blinded to each other's findings, measured each angle twice. Bland-Altman plots were used to calculate mean systematic differences and 95% limits of agreement (LoA). Data were expressed in degrees and as percentages to account for the increase in fetal size with advancing gestation. The impact of measurement variability was evaluated by recalculating the diagnostic accuracy adding to the angle cut-off the intraobserver 95% LoA. RESULTS: Of 132 fetuses diagnosed with suspected antenatal micrognathia, 84 cases had a known outcome (termination of pregnancy (n = 56); fetal demise (n = 11); neonatal death (n = 4); live birth (n = 13)) and 48 were lost to follow-up. Fetal facial profile ultrasound images were available for 49 cases in which reproducibility of the measurements was assessed (termination of pregnancy (n = 22); fetal demise (n = 5); neonatal death (n = 3); live birth (n = 6); lost to follow-up (n = 13)). FNMA was the most reproducible and FMA was the least reproducible measurement. Validation analysis showed that 14%, 100%, 88% and 90% of fetuses from our cohort met the criteria for the diagnosis of micrognathia using the cut-off criteria suggested in the original studies for IFA, FNMA, MNMA and FMA, respectively. The 95% LoA for intra- and interobserver reproducibility were 3.5% and 9.5% for FNMA, and 11.6% and 45.0% for FMA, respectively. When adding the intraobserver widest random error to the original study cut-offs, the percentage of fetuses meeting the criteria for diagnosis decreased to 0%, 87%, 49% and 45% for IFA, FNMA, MNMA and FMA, respectively. CONCLUSIONS: Current quantitative ultrasound methods to assess micrognathia antenatally have variable diagnostic accuracy and are not consistently reproducible, which can lead to overdiagnosis or missed diagnosis. Measurement of FNMA is the preferred method to support subjective assessment, however, further evaluation in prospective studies is warranted. © 2025 The Author(s). Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.