(HealthDay News) – For women whose infants are delivered by vacuum extraction, the risk of obstetric anal sphincter injury (OASIS) is significantly higher among nulliparous women than multiparous women.

Sari Räisänen, PhD, from the Savonia University of Applied Sciences in Iisalmi, Finland, and colleagues investigated the risks of OASIS in a sample of 16,802 nulliparous and multiparous women delivered by vacuum extraction, using a population-based register from Finland, where episiotomy is exclusively lateral. Nulliparous and multiparous women with OASIS were compared with women without OASIS using stepwise logistic regression analysis.

The researchers found the incidence of OASIS to be significantly higher among nulliparous women (3.4%) than multiparous women (1.4%), with an adjusted odds ratio of 2.44. In nulliparous women, lateral episiotomy correlated with a significantly decreased incidence of OASIS (adjusted odds ratio, 0.54), but there was no significant association for multiparous women. There was an increase in the OASIS incidence (2.10-fold for nulliparae and 2.83-fold for multiparae) for a 1,000g increase in birth weight.

“The results of the present study on women delivered by vacuum extraction provide evidence that OASIS risk incrementally decreases with each vaginal delivery,” the authors write. “Lateral episiotomy was associated with an approximately 50% decreased incidence of OASIS in nulliparous women, including women admitted for a first vaginal delivery after a previous cesarean section for their first birth.”

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