The Accuracy of Gestation-Adjusted Projection Method in Estimating Birth Weight by Sonographic Fetal Measurements in the Third Trimester
Sukit Sritippayawan MD*, Wisude Anansakunwat MD*, Chotima Suthantikorn*
Affiliation : * Department of Obstetrics and Gynecology, Charoenkrung Pracharuk Hospital
Objectives : 1. To assess the accuracy of the gestation-adjusted projection method in estimating birth weight in
Thai pregnant women.
2. To determine the efficiency of the gestation-adjusted projection method for the detection of low
actual birth weight, normal birth weight, and large fetus groups.
Study
Design : Diagnostic clinical trial.
Materials and Methods : The present study was conducted on 328 uncomplicated pregnancies that were moni-
tored at the Department of Obstetrics and Gynecology, Charoenkrung Pracharuk Hospital during the period
of August 1 to November 30, 2006. The fetal biometry was measured by ultrasound at 34 weeks’ gestation or
after. Fetal weight was calculated according to Hadlock’s formula. The extrapolation technique is based on
the gestation-adjusted projection (GAP) method. The accuracy of the method was assessed by analyzing the
weight predictions in relation to the actual birth weight (ABW). Main outcome measurements were simple
error, absolute error, absolute percentage error and accuracy within 10% of ABW.
Results : The accuracy within 10% of ABW was 76.5% (95% CI 71.9, 81.1). The estimation tended to be
underestimated (-134.5 + 235.2 grams). The mean of absolute error and of absolute percentage error were
226.2 + 148.8 grams and 7.2 + 4.5% respectively. The smallest observed mean difference was obtained in the
large fetus group (birth weight > 4,000 grams) and the largest one was obtained in the normal birth weight
group (birth weight 2,500-4,000 grams). The accuracy amongst possible contributing factors were compared
and analyzed. The sensitivity and specificity for prediction of birth weight (BW) lower than 2,500 grams (g);
2,500-4,000 g and more than 4,000 g were 60% and 93.8%; 92.5% and 56.5%; 33.3% and 98.8%, respec-
tively. The positive predictive value (PPV) and negative predictive value (NPV) of each BW group were 38.7%
and 97.3%; 96.6% and 36.1%; 20% and 99.4%, respectively. The post-test likelihood when the test is negative
of each BW group was 2.7%, 63.9%, and 0.6%, respectively. The likelihood ratio of a positive result (LR+) and
negative result (LR-) of each BW group were 10 and 0.4; 2.1 and 0.1; 33 and 0.8, respectively. The test
efficiency (TE) of each BW group was 91.7%, 89.9%, and 98.2%, respectively.
Conclusion : The GAP of estimated fetal weight (EFW) is able to accomplish the prediction of BW in Thai
pregnant women with good accuracy. The diagnostic performance of this method for detection of low birth
weight (LBW) and large fetus group are acceptable because of its high specificity, high NPV, high LR+, and
low post-test likelihood ratio when the test is negative.
Keywords : Gestation-adjusted projection method, Sonographic estimation of fetal weight, Birth weight
