662: Association of Montevideo units with uterine rupture in women undergoing a trial of labor after cesarean delivery

662 Association of Montevideo units with uterine rupture in women undergoing a trial of labor after cesarean delivery Lindsay Maggio, Joanna Forbes, Haleh Sangi-Haghpeykar, Christina Davidson Baylor College of Medicine, Obstetrics and Gynecology, Houston, TX OBJECTIVE: To investigate if there is a threshold of uterine contraction strength, as measured by Montevideo units (MVUs), at which catastrophic uterine rupture always occurs and/or never occurs, as well as to determine if the rate of uterine rupture increases as MVUs increase. STUDY DESIGN: Retrospective chart review of women who underwent a trial of labor after cesarean (TOLAC) at a tertiary care academic hospital in Houston, TX, between January 2007 and June 2010. We included women with one prior cesarean delivery (CD) attempting TOLAC at 36 0/7 weeks gestational age or greater with a singleton gestation who had an intrauterine pressure catheter placed during their labor. Cases were those women who experienced a catastrophic uterine rupture during their TOLAC and controls had a successful vaginal birth after cesarean (VBAC). For each case, 6 controls were randomly selected. MVUs were calculated in 10 minute intervals for a total of 120 minutes prior to either delivery (controls) or to the time of planned CD for suspected uterine rupture (cases). Time zero was that time of delivery or planned CD. Cases and controls were compared via chi-square/fisher exact test for grouped variables and t-test for continuous outcomes. Ability of MVU time points to predict uterine rupture was assessed via logistic regression models and the corresponding Area Under the Curve (AUC) and Receiver Operative Characteristic (ROC). RESULTS: Eight cases were identified during the study time period and compared against 48 controls. When comparing MVUs, there was no statistically significant difference between the two groups at any time period except in the zero to ten minute interval (the 10 minutes immediately prior to successful VBAC or planned CD for uterine rupture). At this time point, the mean MVUs in the control group were actually higher than in the cases (271.2 vs. 151.9, p 0.0008), which remains true for almost every other time point, although not statistically significant. CONCLUSION: We did not find a correlation between MVUs and risk of uterine rupture in women undergoing a TOLAC. 663 Predicting cesarean delivery in the second stage of labor: a classification & regression tree (CART) analysis Lorie Harper, Anthony Odibo, George Macones, Alison Cahill Washington University in St. Louis, Department of Obstetrics and Gynecology, St. Louis, MO OBJECTIVE: While uncommon, cesarean delivery (CD) performed in the second stage is associated with increased maternal and neonatal morbidity compared to CD in the first stage of labor. We aimed to develop a prediction model for CD in the second stage of labor using a classification and regression tree (CART) analysis. STUDY DESIGN: A retrospective cohort study of all consecutive women admitted for labor 37.0 weeks gestation from 2004-2008 who reached the second stage of labor. The primary outcome was CD after reaching 10-cm dilation. Univariate and multivariate analyses were used to identify candidate risk factors for CD in the second stage of labor, and CART analysis was performed to identify risk factor scenarios that best discriminate between women who undergo a second stage CD and those who do not. Only factors known at the time a patient reaches 10-cm dilation were used in the analysis. RESULTS: Of 5,388 subjects who reached 10-cm dilation, 88 (1.6%) required CD. The logistic regression model identified 4 risk factors for CD and produced a receiver operator characteristic (ROC) curve with an area under the curve of 0.75. The ROC curve identified no optimal trade-off between sensitivity and specificity for predicting CD. CART analysis identified 6 dominant risk factors applied in 4 specific pathways to predict CD. The most important variable in predicting CD in the second stage was a station higher than 0 at complete dilatation. Pathways predicting no CD resulted in CD rates of 0-0.6%. Pathways that predicted CD had CD rates of 2.5-5.1%. Using this prediction model, 1,791 (34%) of women who delivered vaginally would have been misclassified as requiring CD. CONCLUSION: Despite being able to identify risk factors associated with need for cesarean in the second stage of labor, cesarean delivery in the second stage of labor cannot be reliably predicted based on antenatal and intrapartum characteristics by logistic regression or CART techniques. PosterSessionIV Epidemiology, Infectious Disease, Intrapartum Fetal Assessment, Operative Obstetrics, Obstetric Quality & Safety, Public Health-Global Health www.AJOG.org