A simplified method for determining individual growth curve standards.

To simplify the procedure for obtaining individual growth curve standards, we investigated the use of models derived from the slopes of growth curves determined before 26 weeks' menstrual age. In serial examinations of 33 normal fetuses between 15-38 weeks, we obtained measurements of the biparietal diameter (BPD), head circumference, abdominal circumference, femur diaphysis length, head profile area, abdominal profile area, head cube, and abdominal cube. Rossavik models fitted to these data provided estimates of the coefficients c and s, and fitting linear models to the data before 26 weeks gave estimates of growth curve slopes. Slopes were also estimated from two data points at approximately 16 and 25 weeks. Regression analysis demonstrated a strong linear relationship between loge c and loge SLOPE1 (R2 = 87.3-98.4%), and between s and c (R2 = 64.9-90.3%). Using these relationships, growth models were determined from the slope values. Comparisons between observed measurements and those predicted by these models for the period after 26 weeks indicated that the methods of slope calculation were equally accurate in predicting future growth, and that this accuracy was very similar to that obtained with models based on regression analysis. These results demonstrate that individual growth curve standards for at least eight anatomic parameters during the last 14 weeks of pregnancy can be determined from the data obtained in two examinations before 26 weeks' menstrual age.