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 (β1 = 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.