Fetal ultrasound biometry: 1. Head reference values

Objective To create reliable reference ranges and calculate Z scores for fe tal head ultrasound biometry using a large sample size which is evenly dist ributed from 12 to 42 weeks of pregnancy. Design A prospective, cross-sectional study. Setting Obstetric clinics (outpatient and delivery units) at the University Hospital of Zurich. Sample The study data were obtained from 6557 pregnant women. Methods Only the first ultrasound examination between 12 and 42 weeks of ea ch fetus with exactly established gestational age was used for analysis. No exclusions were made on the grounds of small-for-date birthweight, prematu rity or other events several weeks after the examination. Separate regressi on models were fitted to estimate the mean and standard deviation at each g estational age for each parameter. Results A total of 6217 fetal head biparietal diameters and 5510 occipito-f rontal diameters were measured. Both head circumference and cephalic index were derived in 5462 cases where both biparietal diameter and occipito-fron tal diameter could be measured on the same fetus. The centile charts, table s and regression formulae for biparietal and occipito-frontal diameters, he ad circumference and cephalic index are presented. An application to calcul ate Z scores was developed using Excel (Microsoft Corporation, USA) and mac ros are presented in detail in the Figure 8 footnote. The comparison of our charts with those of the two most recent studies revealed almost no differ ences in biparietal diameter centiles. In one publication, occipito-frontal diameter charts, and in another, head circumference charts were different from the current study. Conclusions We have presented centile charts, tables and regression formula e for fetal head ultrasound biometry derived From a large and minimally sel ected sample size in a carefully designed cross-sectional study. Complete t ables and regression formulae to calculate reference ranges and Z scores ar e presented for use in computer-aided evaluation of fetal ultrasound biomet ry.