Comparison of best landmarks for calculating fetal jaw measurements by ultrasound and MRI in micrognathia

Background: Micrognathia can be diagnosed in utero with ultrasound by measuring the jaw index and/or inferior facial angle, though it can be challenging due to fetal positioning. The jaw index can be measured with magnetic resonance imaging (MRI) using the masseter muscle, but indistinct margins can lead to inaccuracy; the easily visualized posterior teeth buds may be a better landmark.

Objective: We aimed to evaluate inter-reader variability, agreement with ultrasound, and association with postnatal outcomes using MRI to measure the inferior facial angle, jaw index by masseter muscle, and jaw index by posterior teeth buds.

Materials and methods: A single-institution retrospective review was performed of singleton pregnancies with prenatally diagnosed micrognathia by ultrasound or MRI from September 2013-June 2022. Ultrasound measurements were obtained by a maternal-fetal medicine specialist and MRI measurements by two radiologists to evaluate inter-reader variability. Intraclass correlation coefficients (ICC) and Bland-Altman analysis were used to assess agreement between imaging methods and logistic regressions and ROC curves to assess associations with postnatal outcomes.

Results: Forty-three fetuses (median gestational age 26 weeks (IQR 22-31); 47% male (20/43)) were included. Ultrasound measurements could not be obtained for jaw index in 15/43 (35%) fetuses and inferior facial angle in 11/43 (26%); MRI measurements were obtained by at least one reader in all cases. Jaw index by teeth buds demonstrated lowest inter-reader variability (ICC = 0.82, P < 0.001) and highest agreement with ultrasound (bias -0.23, 95% CI -2.8-2.2). All MRI measurements, but not ultrasound, predicted need for mandibular distraction (inferior facial angle P = 0.02, jaw index by masseter muscle P = 0.04, jaw index by teeth buds P = 0.01).

Conclusion: Fetal MRI measurements, particularly jaw index measured by posterior teeth buds, demonstrate low inter-reader variability and high agreement with ultrasound, and may predict need for mandibular distraction postnatally.