Effect of birth weight on adverse obstetric outcomes in vaginal birth after cesarean delivery

To estimate the association between neonatal birth weight and adverse obstetric outcomes in women attempting vaginal birth after cesarean.

We reviewed the medical records of all women undergoing a trial of labor after a prior low transverse cesarean delivery in our institution between 1987 and 2004. Patients were categorized according to birth weight (less than 3,500 g [group 1, reference], 3,500-3,999 g [group 2], and 4,000 g or more [group 3]) and prior vaginal delivery. The rates of failed trial of labor, uterine rupture, shoulder dystocia, and third- and fourth-degree perineal laceration were compared among groups. Multivariable logistic regressions were performed to adjust for potential confounding factors.

Of 2,586 women, 1,519 (59%), 798 (31%), and 269 (10%) were included in groups 1, 2, and 3, respectively. Birth weight was directly correlated to the rate of failed trial of labor (19%, 28%, and 38% for groups 1, 2, and 3, respectively; P<.01), uterine rupture (0.9%, 1.8%, and 2.6%; P<.05), shoulder dystocia (0.3%, 1.6%, and 7.8%; P<.01), and third- and fourth-degree perineal laceration (5%, 7%, and 12%; P<.01). After adjustment for confounding variables, birth weight of 4,000 g or more remained associated with uterine rupture (odds ratio [OR] 2.62, 95% confidence interval [CI] 1.001-6.85), failed trial of labor (OR 2.47, 95% CI 1.82-3.34), shoulder dystocia (OR 25.13, 95% CI 9.31-67.86), and third- and fourth-degree perineal laceration (OR 2.64, 95% CI 1.66-4.19).

Birth weight and specifically macrosomia are linked with failed trial of labor, uterine rupture, shoulder dystocia, and third- and fourth-degree perineal laceration in women who underwent prior cesarean delivery. Estimated fetal weight should be included in the decision-making process for all women contemplating a trial of labor after cesarean delivery.

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