Using anovaginal distance at the beginning of labor to predict the likelihood of instrumental delivery

OBJECTIVE: To investigate whether anovaginal distance in parturients has an impact on the instrumental delivery rate. STUDY DESIGN: In this study with prospective data collection and a convenience sample, 400 parturients planning for vaginal delivery at term with a singleton fetus in vertex presentation were enrolled. Anovaginal distance was measured in the lithotomy position during the latent phase of labor or early first stage. This distance is thought to reflect the angle of inclination of the birth canal axis. RESULTS: At the cutoff point of 25 mm for anovaginal distance, the odds ratio for instrumental delivery was 2.5 (95% CI 1.48-6.6, p < 0.05). A final model of instrumental delivery, controlling for various confounding variables, correctly predicted 91.38% of the instrumental deliveries (p < 0.0001). The variables anovaginal distance > 25 mm (OR 2.9,95% CI 1.2-6.7, p = 0.01), parity (OR 0.54, 95% CI 0.29-0.99, p = 0.046) and duration of second stage of labor (OR 1.01, 95% CI 1.004-1.01, p = 0.001) significantly contributed to the prediction of instrumental delivery after controlling for the other parameters. CONCLUSION: Women with an anovaginal distance of > 25 mm at the beginning of labor was 2.5 more likely to undergo instrumental delivery than were women with an anovaginal distance of < 25 mm.