Objective: To provide an objective and accurate tool to diagnose micrognathia in the fetus. Methods: The anteroposterior and laterolateral diameter of the mandible were measured in 262 normal fetuses between 12 and 37 weeks’ gestation and plotted against gestational age and biparietal diameter (BPD). The jaw index (anteroposterior mandibular diameter/BPD × 100) was then tested against the usual subjective method for diagnosing micrognathia, consisting of the evaluation 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% sensitivity and 98.1% specificity in diagnosing micrognathia, in comparison with 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 growth and gestational age or BPD. In addition, we validated the jaw index as an objective tool for diagnosis of micrognathia in the fetus.