Study of accuracy of commonly used fetal parameters for estimation of gestational age.

Estimation of the gestational age by sonographic measurements of fetal parameters is usually done by measuring mean sac diameter (MSD), Crown-rump length (CRL), biparietal diameter (BPD), head circumference (HC), femoral length (FL) and abdominal circumference (AC) depending on the trimester of pregnancy. This is a prospective study to see the correlation of different fetal parameters in estimating the gestational age. A total of 71 normal women were taken and ultrasonogram was done to take MSD and CRL in 1st trimester and BPD, HC, FL and AC in 2nd and 3rd trimester. Gestational age was calculated by MSD using Rempen normogram and Hadlock normograms were used for the rest of fetal parameters. Gestational age by LMP was also calculated at the same time. Correlation of different fetal parameters in estimating gestational age in different trimesters was done by Pearson correlation. Pearson correlation showed that the CRL was the best fetal parameter (correlation coefficient of 0.909) in the first trimester. The correlation of MSD and average gestational age by MSD and CRL was with correlation coefficients of 0.778 and 0.888 respectively. Thus the averaging of gestational age in 1st trimester decreases the accuracy in the gestational age estimation. Correlation of gestational age by BPD, HC, FL and AC and their average in 2nd trimester showed that the best correlation was by AC and the least correlation by BPD in this study. It is in contrary to other studies which could be due to chance finding or bias because of prospective study. Pearson correlation calculated in 3rd trimester showed HC and FL were better parameters than BPD and AC. Average gestational age by simple averaging of BPD, HC, FL and AC gives more accurate estimation of gestational age in both 2nd and 3rd trimester.