Comparison of Johnsons, Inslers formula and ultrasound for estimating fetal weight

Accurate estimation of fetal weight is a crucial component of pregnancy management and plays a key role in reducing maternal and neonatal complications. Although ultrasonography is considered the gold standard, its limited availability and cost emphasize the need to assess alternative clinical methods.

This cross-sectional study included 132 term pregnant women (37to 41 weeks) selected via non-probability sampling. Fetal weight was estimated prior to childbirth using three methods: ultrasonography (performed within the last two weeks of the third trimester to minimize the impact of rapid fetal weight gain closer to term), Johnson's formula, and Insler's formula and was compared with the actual birth weight immediately after childbirth. Data were analyzed using SPSS version 24, with descriptive statistics and regression.

The mean ± SD of estimated fetal weights (EFW) obtained via third-trimester ultrasonography and based on the Johnson and Insler formulas were 3164 ± 382.95 g, 3161.27 ± 302.90 g, and 3309.35 ± 374.05 g, respectively. Regarding diagnostic accuracy, the Johnson formula showed a precision of 69.70% for neonates < 3000 g and 71.21% for macrosomia (> 3500 g), the Insler formula showed a precision of 75.76% for neonates < 3000 g and 68.94% for macrosomia (> 3500 g). In the multivariate regression analysis of estimated fetal weight using the Johnson formula, only fundal height remained a significant independent predictor (β = 149.43, P < 0.001).

Although ultrasound is the most accurate method for estimating fetal weight, with the smallest mean difference from the actual weight, clinical methods, particularly the Insler method for normal weight pregnancies and the Johnson method for ruling out macrosomia, have acceptable accuracy.

Not applicable.