Objectives: To discuss the features of study design and analysis which are necessary to derive valid reference centiles for fetal size. To describe a study which meets the stated criteria.
Design: Prospective study of 663 fetuses.
Results: The selection of the sample and adequate sample size are of great importance. Prospective collection of data specifically for the purpose of deriving centiles is recommended. It is essential to use statistical methods that take proper account of the increasing variation among fetuses as pregnancy proceeds; such methods are described and illustrated. A study is described which meets the stated criteria for design and analysis, and from which new fetal size centile charts have been derived and are presented in subsequent papers.
Conclusions: Many published studies containing charts (standards) of fetal size are methodologically flawed. Research design and statistical analysis must adhere to sound principles for fitted centiles of size to be valid and so clinically relevant.