Optimal standards for fetal biometry: to each measurement its fitting model

Objective: To determine the best mathematical model to construct charts of fetal abdominal circumference (AC) and femur length (FL). Methods: Ultrasound measurements were made on 1,336 normal fetuses in one center. Four mathematical models were compared (a linear-quadratic model, a linear-cubic model, the Rossavik model and a new two-phase model, which has been found to best fit fetal head data). Results: The best fitting of AC and FL data was obtained with the linear-quadratic model without separate computing for gender. Centile charts have been computed. Conclusion: Fetal growth of AC and FL is much simpler than that of the head. Therefore, a unique mathematical model should not be used to fit all measurements