The diagnosis should be confirmed using specific indices [5, 6], ratios or facial angles [7, 8].
Inferior facial angle—This angle is measured in the midsagittal view of the fetal profile and is formed by the crossing of a line orthogonal to the vertical part of the forehead drawn at the level of synostosis of nasal bones and a line through the tip of the mentum and the more protrusive lip, usually the upper lip (Fig. 1). Its reported normal mean value is 65° ± 16°, so that the upper and lower reference limits are from 81° to 49°. Thus, an angle less than 49° gives the diagnosis of micrognathia, with a sensitivity and specificity of 100% and 99% respectively. The positive predictive value of the inferior facial angle is reportedly 75% and the negative predictive value is 100% .
Jaw index—The jaw index is calculated after measuring the anteroposterior diameter of the fetal mandible, at the axial plane, and referring it as a percentage of the biparietal diameter, providing an index that is independent of gestational age (Fig. 2). The anteroposterior diameter is the distance between the symphysis mentis and the middle of the line connecting the bases of the two rami (axial diameter). Using a cut-off value of 23 mm (2 SD below mean) the sensitivity and specificity is reportedly 100% and 98% respectively . The positive predictive value to predict micrognathia is 69% and the negative predictive value is 100% . The mean value of anteroposterior and axial mandibular diameter is 19 ± 2.3(SD) mm and 13 ± 1.2(SD) mm respectively, at 18 weeks of gestation and 35 ± 2.8(SD) mm and 27 ± 2.3(SD) mm respectively, at 28 weeks of gestation .
Frontal nasomental angle—This is the angle between the line drawn from the tip of the nose and frontal bone, intersecting the line from the nasal tip to the mentum (Fig. 3). Its normal mean value is 147° ± 2.7°(SD) so that the upper and lower reference limits are 142° to 152°. Thus, an angle less than 142° is consistent with the diagnosis of micrognathia . This index is reported to have a lower positive predictive value . Many normal fetuses may have a nasomental angle below the fifth centile. According the recent literature, the frontal nasomental angle in pathological cases ranges from 100° to 134°.
Mandible width/maxilla width ratio—the measurements are obtained on axial views at the alveolar level and 10 mm posterior to the anterior osteous border  (Fig. 4). This ratio is found to be constant over the second trimester. The mean value of this ratio is 1.02 ± 0.12 (SD) Consequently, a ratio less than 0.78 (below the 5th centile) is used to define micrognathia .
Mandibular length—Mandibular length adjusted for gestational age or fetal biometry (like femur length), seems to be another sensitive and reliable prenatal method for assessment of fetal jaw development [5, 6, 9, 12]. Specific charts provide mean values and 95% intervals for mandibular length according gestational age or femur length . When measuring the mandibular length the proximal landmarks are the cartilaginous symphysis menti and the temporomandibular joint  (Fig. 5). The increase in mandibular length is almost linear with increasing gestational age and varies from approximately 20 mm at 18 weeks to 37 mm at 28 weeks . Measurements below the 5th centile are suggestive of micrognathia .
Detection of fetal micrognathia relatively early in the first trimester is feasible, partly due to the advances in fetal imaging. Apart from the assessment of the fetal profile, a useful sonographic sign is the mandibular gap in the retronasal triangle view (Fig. 6). The retronasal triangle view is the coronal plane of the face that captures the primary palate and the frontal processes of the maxilla simultaneously. In a normal fetus, a characteristic gap between the right and left body of the mandible is visible in this view . The mandibular gap is linearly increased with CRL and progressive ossification of the facial bones allows easier identification. On the other hand, shadowing from the primary palate can make visualization challenging. It is advised that all suspicious cases are reassessed later on in the second trimester, especially in the absence of major chromosomal and/or structural abnormalities.
3D sonographic views can add to the diagnosis, but the evaluation is mainly subjective. Prenatal MRI, on the other hand, may be a useful adjunct to ultrasound for the diagnosis and postnatal surgical management of craniofacial abnormalities . Its diagnostic value is even higher in cases of limited acoustic window, maternal obesity, oligohydramnios and anterior spine position . As the mandibular body grows more rapidly than the ramus and thus more rapidly in the longitudinal rather than the vertical plane, micrognathia, that is abnormal growth, primarily impairs longitudinal growth. Therefore, anteroposterior diameter measurements are more appropriate to assess the mandibular growth than other measurements .
One should be careful in pronouncing a fetus having ‘micrognathia’, especially on subjective evaluation. This term implies that the fetus is abnormal with presence of significant pathology. Micrognathia may be less apparent with continued growth and development. There is no ‘gold standard’ for a definitive diagnosis of micrognathia on post-natal evaluation. Using a combination of objective sonographic markers as well as follow-up ultrasound assessments can significantly reduce the risk of a false diagnosis.