Investigating the accuracy of Johnson’s rule in estimating fetal weight

Abstract

BACKGROUND: Birth weight has a significant impact on perinatal mortality. Therefore, the estimation of fetal weight greatly influences the policies necessary for care during and after delivery. We aimed to investigate Johnson’s rule in estimating fetal weight. MATERIALS AND METHOD: This study was a single-group longitudinal study that was conducted in 6 months from October 2021 to April 2022 on 150 pregnant women in Isfahan-Iran. The sampling method was accessible. Inclusion criteria include being term, singleton, without abnormality, intact membranes, cephalic presentation, and exclusion criteria include diagnosed polyhydramnios or oligohydramnios and mother’s abdominal or pelvic known masses. After completing the informed consent, fetal weight was estimated by Johnson’s rule and was compared with the birth weight. Descriptive and analytical statistics (mean-standard deviation (SD), number-percentage, t-paired, and Spearman’s correlation coefficient) were used to achieve the objectives of the study. The receiver operating characteristic (ROC) curve was also used to determine the sensitivity, specificity, and positive and negative predictive value of Johnson’s law. RESULT: The mean (SD) birth weight was 3032.88 ± 481.11 g and the mean (SD) estimated fetal weight (EFW) by the clinical method was 3152.15 ± 391.95 g. There was a significant difference between the averages (P < 0.001). The percentage error of EFW showed a significant negative correlation (r = -0.286; P < 0.05) with gestational age (GA) and a significant positive correlation (r = 0.263; P < 0.05) with the fetal head station. The sensitivity and specificity of EFW with Johnson’s rule, in normal fetal birth weight, were higher than in low birth weight fetal. The accuracy of EFW with ± 10% of the actual weight was higher in average for gestational age (AGA) (84.3%) and high-for-gestational-age (LGA) (70%) than in low-for-gestational-age (SGA) (4%). The EFW mean percentage error in SGA was higher than in the other two weight groups. This method, especially for AGA and LGA fetuses, can be a suitable alternative to other weight estimation methods. CONCLUSION: Clinical estimation of weight via Johnson’s rule due to availability and no cost can be a suitable method for managing childbirth based on fetal weight.