NORMAL INTERVAL FETAL GROWTH-RATES BASED ON OBSTETRICAL ULTRASONOGRAPHIC MEASUREMENTS

Analyzing fetal interval growth may improve detection of growth abnorm alities. This study establishes normal limits for growth rates of feta l biparietal diameter, abdominal diameter, abdominal circumference, an d femur length. Fetal measurements were obtained during 1450 obstetric al sonographic examinations in 510 patients with normal-weight term de liveries. Using a random-effects model for longitudinal data, cubic cu rves of size versus mean gestational age were fitted using a restricte d maximum likelihood technique. Quadratic growth rate curves were then calculated as derivatives of the fitted curves. The growth rate equat ions are as follows: Biparietal diameter growth rate = 2.59 + 0.127 (M GA)-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 gestati onal age in wk. Because standard deviations depend on the time interva l between examinations, separate growth rate tables are provided for i ntervals of 4, 6, 8, and greater than or equal to 10 weeks.