Growth standards for anatomic measurements and growth rates derived from longitudinal studies of normal fetal growth

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Abstract
A statistical procedure for deriving growth standards for anatomic measurements and their growth rates from longitudinal studies of fetal growth was evaluated using Rossavik growth models for the biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur diaphysis length (FDL) determined in a previous study of normal fetal growth. For each anatomic parameter, the coefficients c and s of the model was used to define a set of growth curves that constituted the boundary growth curves of a region containing 95% of the growth curves of this data set. The set of boundary growth curves was used to specify the mean, lower limit, and upper limit values for the anatomic parameter and its growth rate at weekly intervals between 14 and 38 weeks, menstrual age. Comparison of these values to those determined from cross‐sectional studies of fetal growth gave differences of −1.9% to 4.8% (SD: ±0.9 to ±2.6) for mean vs. predicted value of the anatomic measurements. For the lower limit, similar values were 0.4% to 13.8% (SD: ± 1.7 to ±8.8); for the upper limit the values were 8.3% to 18.0% (SD: ±1.5 to ±7.0). Comparisons of HC growth rates determined using polynomial and Rossavik growth models gave values of −3.4% (SD: ±4.4) for mean vs. predicted value, −12.6% (SD: ±10.6) for the lower limit and 5.2% (SD: ±9.3) for the upper limit. The degree of agreement was similar for AC growth rates. These results indicate that reasonable growth standards for anatomic measurements and their growth rates can be determined from longitudinal studies of as few as 20 normal fetuses, although better estimates of normal variability could be obtained with a larger sample.