CHARTS OF FETAL SIZE .1. METHODOLOGY

Objectives To discuss the features of study design and analysis which are necessary to derive valid reference centiles for fetal size. To de scribe a study which meets the stated criteria. Design Prospective stu dy of 663 fetuses. Results The selection of the sample and adequate sa mple size are of great importance. Prospective collection of data spec ifically for the purpose of deriving centiles is recommended. It is es sential to use statistical methods that take proper account of the inc reasing variation among fetuses as pregnancy proceeds; such methods ar e described and illustrated. A study is described which meets the stat ed criteria for design and analysis, and from which new fetal size cen tile charts have been derived and are presented in subsequent papers. Conclusions Many published studies containing charts (standards) of fe tal size are methodologically flawed. Research design and statistical analysis must adhere to sound principles for fitted centiles of size t o be valid and so clinically relevant.