Predicting obstetric anal sphincter injuries (OASIS) in pati
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Women with a history of prior cesarean section must weigh the numerous potential risks and benefits of elective repeat cesarean section or trial of labor (TOLAC). One important risk of vaginal delivery is obstetric anal sphincter injury (OASIS), and it occurs at higher rates among women undergoing vaginal birth after cesarean (VBAC).

This was a secondary subgroup analysis of the Maternal Fetal Medicine Units Network (MFMU) TOLAC prospective cohort. We identified women within the MFMU cohort with one prior cesarean section followed by a term VBAC. This cohort was stratified into two groups based on presence of OASIS and baseline characteristics were compared with bivariate analysis.

There were 10,697 women in the MFMU TOLAC cohort who met inclusion criteria, and 669 (6.3%) sustained OASIS. Factors independently associated with OASIS included in the model were use of forceps (aOR 5.08, 95% CI 4.10-6.31) and vacuum (aOR 2.64, 95% CI 2.02-3.44) assistance, along with increasing maternal age (aOR 1.05, 95% CI 1.04-1.07 per year), BMI (aOR 0.99, 95% CI 0.97-1.00 per unit kg/m2), prior vaginal delivery (aOR 0.19, 95% CI 0.15-0.23), and tobacco use during pregnancy (aOR 0.59, 95% CI 0.43-0.82). Internal validation demonstrated appropriate discrimination (concordance index 0.790, 95% CI 0.771-0.808) and calibration (Brier score 0.047). External validation utilized data from 1,266 women delivering at a tertiary healthcare system, with appropriate model discrimination (concordance index 0.791, 95% CI 0.735-0.846) and calibration (Brier score 0.046). The model can be accessed at oasisriskscore.

Our model provides a robust, validated estimate of probability of OASIS during VBAC using known antenatal and one modifiable intrapartum risk factor and can be utilized to counsel patients regarding risks of TOLAC as compared to elective repeat cesarean section.

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