Predictors of obstetric anal sphincter injury during waterbirth: a secondary analysis of a prospective observational...

Obstetric anal sphincter injury (OASI) during childbirth is associated with urino-genital pain and dysfunction. Waterbirth is a popular birth choice for women, but controversy remains around the risk of OASI during waterbirth. This study reports on the incidence of OASI, and factors associated with OASI, for a cohort of women who gave birth in water.

This secondary analysis used prospectively collected data from 2,908 women who gave birth in water in a hospital setting. The incidence of OASI was calculated. Univariable and multivariable logistic regression analysis evaluated factors associated with OASI.

The incidence of OASI was 1.9% (95% confidence interval (CI) 1.4, 2.4) for all women. In nulliparae it was higher (3.2%, 95% CI 2.3, 4.3) than in multiparae (0.9%, 95% CI 0.5, 1.4). In the multivariable analysis, two variables were associated with OASI; multiparity was negatively associated with OASI (adjusted odds ratio [aOR] 0.24, 95% CI 0.12, 0.50, p < 0.001), and birth weight was positively associated with OASI (aOR 1.001, 95% CI 1.000, 1.002, p = 0.02). A "hands-on" technique was used during only 13% of births. A birth position supporting a flexible sacrum did not influence OASI risk.

A low incidence of OASI was found for this cohort of women. The low proportion of midwives using a hands-on technique suggests that it may not be required in waterbirth.