Reference ranges of fetal mandible measurements: Inferior facial angle, jaw index, mandible width/maxilla width ratio and mandible length in Thai fetuses at 15 to 23 weeks of gestation

OBJECTIVES: To determine the normal distribution of 1) inferior facial angles (IFA), 2) jaw index, 3) mandible width/maxilla width ratio (MD/MX ratio), and 4) mandible length (ML) in second trimester Thai fetuses. METHODS: A prospective study was performed between April 1 and October 31, 2020, at the Maternal-Fetal Medicine Unit of Songklanagarind Hospital. Transabdominal ultrasonography was performed on Thai singleton pregnant women at 15(0/7) to 23(6/7) weeks of gestation to measure IFA, jaw index, MD/MX ratio and ML. All women received standard antenatal care and were followed up until delivery. The exclusion criteria were multifetal gestation, congenital anomaly, chromosomal abnormality, fetal growth restriction, abnormal amniotic fluid volume, suspected abnormality of fetal mandible, maxilla or jaws based on the proposed criteria from previous studies and suspected neonatal structural or genetic abnormalities postnatally. Quantile regression was used to estimate changes in the median, 5th and 95th percentiles of each parameter across gestational ages and to generate formulas for predicting the 5th percentile value for each parameter. RESULTS: The results of 291 women were analyzed. Scatter plots and reference ranges of each parameter were generated. IFA, jaw index and ML values significantly increased, while the MD/MX ratio value significantly decreased, with increasing gestational age. The formulas calculated for predicting the 5th percentile value for each parameter were IFA = 55.12 + 0.045*(GA in days—136) jaw index = 37.272 + 0.01693*(GA in days—136) MD/MX ratio = exp(0.027–0.00110*(GA in days—136)) ML = 20.83 + 0.243*(GA in days—136). CONCLUSIONS: The reference ranges and formulas to calculate the 5th percentile values of mandible parameters in Thai fetuses were developed. TRIAL REGISTRATION: This study has been reviewed and approved by the Thai Clinical Trials Registry with identification number TCTR20210602003.