Effect of membrane rupture on the accuracy of Johnson–Toshach clinical fetal weight estimation at term: A prospective intra‐subject study

Objective To evaluate the effect of membrane rupture on the agreement between fetal weight estimated using the Johnson–Toshach method and birth weight at term, and the influence of examiner experience and amniotic fluid volume. Methods A prospective intra‐subject observational study was conducted in 179 women aged 18–39 years with term singleton pregnancies admitted in labor at a teaching hospital in Mexico. Fetal weight was estimated using the Johnson–Toshach method before and after membrane rupture and compared with birth weight. Agreement was assessed using Lin's concordance correlation coefficient (CCC) and mean absolute error (MAE). Bland–Altman analysis evaluated bias and limits of agreement. Secondary analyses examined examiner experience and the association between amniotic fluid index (AFI) and changes in fundal height after membrane rupture. Results Agreement between estimated fetal weight (EFW) and birth weight improved after membrane rupture. The CCC increased from 0.577 (95% CI 0.464–0.673) before rupture to 0.710 (95% CI 0.583–0.810) after rupture. MAE decreased from 286 to 178 g, with a paired reduction of 108 g (95% CI 75–143 g; P < 0.001). Estimation error decreased with increasing examiner training level. Each 1‐cm increase in AFI was associated with a 0.12‐cm greater reduction in fundal height after membrane rupture ( β 1 = 0.116, 95% CI 0.088–0.144; P < 0.001). Conclusion Membrane rupture improves the accuracy of fetal weight estimation using the Johnson–Toshach method at term. Examiner experience and amniotic fluid volume also influence estimation accuracy.