Objective diagnosis of micrognathia in the fetus: The jaw index

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.