Prenatal indices for mandibular retrognathia/micrognathia

Micro- and retrognathia of mandibular origin may lead to life-threatening respiratory problems in connection with glossoptosis immediately after birth. Prenatal screening for this malformation is therefore increasingly important. Today this is accomplished by predominantly subjective standards. Objective criteria have been proposed but have not become established. We therefore made an effort to develop indices that would identify major skeletal discrepancies or micrognathia in as straightforward a fashion as possible during routine prenatal sonography.

Series of fetal jaw sonograms (Toshiba Aplio MX®) were obtained in 313 women with normal pregnancies from weeks 19-29 of gestation. Upper- and lower-jaw landmarks were interactively located on screen and evaluated for reproducibility. Linear parameters representative of maxillary and mandibular length (SpA'-SpP' and Rami-SymMe) were measured and related to femur length and gestational age. Based on these data, indices for maxillary, and mandibular length were derived and analyzed.

High correlations were identified for mandibular length both with gestational age (R = 0.845; R(2) = 0.713) and with femur length (correlation coefficients (R) = 0.839; coefficients of determination (R(2)) = 0.704). For maxillary length, the respective correlation coefficients were 0.691 (R(2) = 0.477) and 0.656 (R(2) = 0.430). Estimates of mandibular and maxillary length based on gestational age and femur length were obtained by regression analysis. The mean bimaxillary length ratio was 0.628 ± 0.043.

Maxillary and mandibular growth can be objectively evaluated via indices. It is conceivable to develop this approach into a sensitive and reliable method of prenatal jaw screening for major skeletal anomalies and congenital malformations.