Using ultrasonic fetal size measurements to estimate gestational age in Brisbane, Australia.

The purpose of this paper was to construct population-specific charts of gestational age (GA) from antenatal ultrasound biometry assessment using a large sample of normal Australian pregnancies when examination was carried out to a standard protocol by experienced operators. All consenting eligible women attending a large Brisbane clinic between January 1993 and April 2003 with GA between 15 and 41 weeks, determined before examination from the last menstrual period date and concordant with the biometric-derived gestation published within the Australian Society for Ultrasound in Medicine (ASUM) policies and statements, were recruited into the prospective study if fulfilling the inclusion criteria. Biparietal diameter, femur length, abdominal circumference and head circumference measurements were recorded using ASUM standard protocol for mid and third trimester obstetric morphology scans. Statistical analyses were carried out using polynomial regression models and thorough diagnostic checks were undertaken. Included within the study were separate scans for 20,555 pregnancies from 17,660 women. Equations, means and 95% reference intervals for GA were derived for each sonographic measurement. These new population-specific regression equations complement those previously published in the same sample of Australian pregnancies. Once validated, we believe they should form the basis of a new Australian standard for ASUM.