(1148) Nomogram for Predicting Obstetric Anal Sphincter Injuries in Nulliparous Women

To develop a predictive model for obstetric anal sphincter injuries (OASIS) in nulliparous women.

Study Design:

This retrospective observational study analyzed data from a tertiary care center. Inclusion criteria encompassed all singleton deliveries at ≥ 37 gestational weeks from January 2007 to December 2019, involving nulliparous women. The primary outcome was the occurrence of OASIS. The full multivariate logistic regression included variables such as age, body mass index (BMI), mode of delivery (MOD) (vaginal or vacuum-assisted), use of oxytocin, spontaneous or induced labor, birth weight, neuraxial analgesia, and episiotomy. The stepwise model, selected based on the Akaike Information Criterion, retained the variables BMI, MOD, induction, and birth weight. A nomogram was then developed using the best-fitting model to visually represent the predicted probability of OASIS. The nomogram was calibrated and validated internally to ensure accuracy and reliability.

Results:

The study cohort comprised 22,738 deliveries: 17,518 vaginal deliveries (77%) and 5,220 vacuum-assisted deliveries (23%). Overall, 221 cases (1%) were diagnosed with OASIS. The variables significantly associated with the primary outcome in the multivariate model were BMI (p = 0.034), MOD (p < 0.001), induction of labor (p = 0.010), and birth weight (p < 0.001). Their respective predicted probabilities are represented in the nomogram (Figure 1).

Conclusion:

The developed nomogram allows for the calculation of individual risk scores for OASIS, identifying patients with a very high risk for OASIS (above 10%) versus those with low risk (less than 1%).