To characterize the growth of the thigh circumference (ThC) in individual fetuses, longitudinal studies of ThC growth were carried out in 20 fetuses between 19
( 2 1.8) and 38 ( * 1.5) weeks, menstrual age. Because of measurement uncertainties, analysis of growth patterns was limited to the data collected after 22 weeks. The Rossavik growth model fit the data well [R2: 95.8( 2 2.9 SD)%l, but considerable coefficient variability was seen, particularly for the coefficient K. Use of the coefficient Kvalue (1.138) derived from a cross-sectional data set reduced the variability of the coefficient c by 95% and made the results of the longitudinal and cross-sectional studies consistent with each other. This K-value was taken as the appropriate one for ThC growth curves. Studies of ThC growth in individual fetuses revealed considerable variability in growth curve shape, although the majority of curves could be approximated by a straight line. No differences between growth curves for males and females could be detected. The average longitudinal ThC growth curve was found to be very similar to the growth curve obtained in a cross-sectional study of ThC growth. ThC growth after 26.1 weeks could be predicted with an accuracy of ? 15% from growth models derived from the data obtained before 26.1 weeks. These results indicate that ThC growth after 22 weeks (as with other parameters) can be followed in individual fetuses. However, because of the greater inherent variability in growth patterns, using each fetus as its own control may be more important. Indexing Words: Bicircumference .
Individual growth curve standards
The fetal thigh circumference (ThC) has been proposed as a indicator of soft tissue mass,' and its growth during pregnancy has been investigated in several cross-sectional ~t u d i e s . ~-~ There have been no previous longitudinal studies of ThC growth. While useful in evaluating the growth characteristics of a population, cross-sectional studies provide no information on ThC growth in individual fetuses and do not permit the development of individual growth curve standard^.^,^ This information can only be obtained from longitudinal studies of ThC growth.
Characterizing ThC growth in individual fetuses requires the use of mathematical modeling and in this investigation the model presented by From the Departments of Obstetrics/Gynecolog,* Pediatrics,t and Radiology,$