Normal interval fetal growth rates based on obstetrical ultrasonographic measurements

Analyzing fetal interval growth may improve detection of growth abnormalities. This study establishes normal limits for growth rates of fetal biparietal diameter, abdominal diameter, abdominal circumference, and femur length. Fetal measurements were obtained during 1450 obstetrical sonographic examinations in 510 patients with normal-weight term deliveries. Using a random-effects model for longitudinal data, cubic curves of size versus mean gestational age were fitted using a restricted maximum likelihood technique. Quadratic growth rate curves were then calculated as derivatives of the fitted curves. The growth rate equations are as follows: Biparietal diameter growth rate = 2.59 + 0.127 (MGA) - 0.000447 (MGA)2; abdominal diameter growth rate = 3.61 + 0.0325 (MGA) - 0.0017 (MGA)2; abdominal circumference growth rate = 11.3 + 0.102 (MGA) - 0.00534 (MGA)2; and femur length growth rate = 5.49 - 0.17 (MGA) + 0.00181 (MGA)2. Rates are expressed in mm/wk and mean gestational age in wk. Because standard deviations depend on the time interval between examinations, separate growth rate tables are provided for intervals of 4, 6, 8, and > or = 10 weeks.