Diagnosis

Until recently, micrognathia was diagnosed by the subjective impression of a small jaw seen on examination of the fetal facial profile. Hence subtle degrees of this disorder were easily missed and diagnosis often delayed until the late 2nd and 3rd trimesters. Nomograms with centile charts of measurements of the fetal mandible now allow greater objectivity in the prenatal diagnosis of this craniofacial abnormality. Measurement of the fetal mandible in the 2nd trimester can be accomplished if the orbits are first identified in transverse section with angling of the transducer caudad to identify the temporomandibular joint. On rotation of the transducer the maximal length of the mandible can be recognised and measured to the symphysis menti. Reliability of this measurement is diminished in the 3rd trimester, often due to technical difficulties arising from fetal crowding, descent of cephalic pole into the pelvis, and acoustic shadowing from adjacent bony structures. Prevalence of antenatally diagnosed micrognathia in the low risk population remains unknown, although in the neonate frequency is said to approach 1%. In referred populations investigated for suspected abnormalities, micrognathia was the most frequently confirmed craniofacial anomaly and was associated with chromosomal abnormality in 38% to 66% of fetuses. Skeletal dysplasias are frequently recognised concurrent abnormalities; some syndromes may include abnormalities of other systems, VSD, splenic, gonadal, lung defects.