Objective: To provide an objective and accurate tool to diagnose micrognath
ia in the fetus.
Methods: The anteroposterior and laterolateral diameter of the mandible wer
e measured in 262 normal fetuses between 12 and 37 weeks' gestation and plo
tted against gestational age and biparietal diameter (BPD). The jaw index (
anteroposterior mandibular diameter/BPD x 100) was then tested against the
usual subjective method for diagnosing micrognathia, consisting of the eval
uation of the facial profile, in a population of 198 malformed fetuses, 11
of which had micrognathia at necropsy or birth.
Results: The mandibular growth was linearly correlated with gestational age
and BPD. Using a cutoff level of less than 23, the jaw index had a 100% se
nsitivity and 98.1% specificity in diagnosing micrognathia, in comparison w
ith 72.7% and 99.2% shown by the subjective evaluation of the fetal profile
. With a cutoff of 21, it yielded a positive predictive value of 100%.
Conclusion: We demonstrated the linear relationship between mandibular grow
th and gestational age or BPD. In addition, we validated the jaw index as a
n objective tool for diagnosis of micrognathia in the fetus. (C) 1999 by Th
e American College of Obstetricians and Gynecologists.