Evaluation of first trimester ultrasound fetal biometry ratios femur length/biparietal diameter, femur length/abdominal circumference and femur length/foot for the screening of skeletal dysplasia

OBJECTIVE: Skeletal dysplasia is usually apparent in the second trimester. First trimester femur-length/biparietal diameter (FL/BPD), FL/abdominal circumference (AC) and FL/foot for the screening of skeletal dysplasia were evalauted. METHOD: Case-control study: pregnancies with molecular confirmation of skeletal dysplasia undergoing nuchal translucency (January_2007-December_2021). Controls included pregnancies without fetal abnormalities. Performance of FL/BPD, FL/AC and FL/foot was evaluated by receiver operating characteristic curves. RESULTS: Twenty-eight skeletal dysplasia cases were identified; 17 (60.71%) corresponded to lethal types. Compared to 184 controls, cases had a lower median FL/BPD (0.34 [IQR 0.30-0.38] vs. 0.44 [IQR 0.39-0.48]; p < 0.001), FL/AC (0.13 [IQR 0.09-0.15] vs. 0.15 [IQR 0.13-0.16]; p = 0.001) and FL/foot (0.84 [IQR 0.76-0.91] vs. 1.01 [IQR 0.94-1.11]; p < 0.001). FL/BPD and FL/foot ratios had a superior area under the curve (0.846 and 0.853, respectively) than FL/AC (0.64). The probability of diagnosing skeletal dysplasia increased at least 9-fold if FL/BPD <0.376 (OR 26.05, 95% CI 9.79-69.3; p < 0.001) and 14-fold if FL/foot <0.891 (OR 39.46, 95% CI 14.17-109.9; p < 0.001). Low FL/BPD and FL/foot were associated significantly with lethal types compared to viable skeletal dysplasia. CONCLUSIONS: First trimester FL/BPD and FL/foot may be of clinical utility in the detection of skeletal dysplasia especially when there is another suspicious sonographic sign or when there is a relevant family history before overt second trimester sonographic markers become apparent.