Reliability of ultrasound fetometry in estimating gestational age in the second trimester.

The reliability of ultrasound fetometry for estimating gestational age (GA) in the second trimester was evaluated, using the fetal variables: biparietal diameter (BPD), occipitofrontal diameter (OFD), mean abdominal diameter (AD), and femur length (FL), each value being taken as the mean of five measurements. Of the individual variables, BPD gave the best precision, with a standard deviation (SD) from true GA of 3.2 days. Using a combination of all four variables, GA could be estimated with a SD of 2.2 days, which was not significantly better than the formula, GA = BPD X 1.2 + FL X 1.0 + 49, which gave results with a SD of 2.4 days. Equations obtained from regression analysis of the variables against true GA were tested in 44 cases where the precise date of conception was known, GA being estimated by BPD with a SD of 3.2 days, and by the combination of BPD and FL with a SD of 2.7 days; the maximum difference between GA estimated by BPD and by FL was 7 days. Using a combination of BPD and FL to estimate GA in the total population, the number of post-term deliveries was only marginally less than when using BPD alone.