Trends in major risk factors for anal sphincter lacerations: a 10-year study.

OBJECTIVES: (1) To identify independent risk factors for anal sphincter laceration, (2) to determine the trend in rates of anal sphincter laceration over a 10-year period, and (3) to examine the impact of temporal trends in risk factors on anal sphincter laceration rates.
METHODS: Population-based data were obtained from the Nova Scotia Atlee Perinatal Database, on 91 206 women who had a singleton vaginal live birth > or =500 g for the years 1988 to 1997. Risk factors for anal sphincter laceration were identified using stepwise logistic regression. A multivariate model was used to study temporal changes in laceration rates after controlling for changes in parity, episiotomy rates, operative vaginal deliveries, birth weight, prolonged second stage of labour, and other determinants.
RESULTS: Nulliparity (relative risk [RR] = 6.97), occiput posterior position (RR = 2.44), non-vertex presentations (RR = 2.27), second stage > or =120 min (RR range = 1.47-2.02), delivery by an obstetrician (RR = 1.30), and birth weight > or =3000 g (RR range = 1.43-6.63) increased the risk of laceration. Instrument-assisted delivery involved risks that ranged from a 2-fold increase for a vacuum-assisted delivery (RR = 2.15) to a greater than 5-fold increase for a forceps delivery after an unsuccessful vacuum extraction (RR = 5.69). Episiotomy, particularly midline incisions, increased the risk of laceration (RR = 2.57). The risk of a sphincter laceration increased 2-fold from 1988 to 1997, despite controlling for risk factors.
CONCLUSIONS: Sufficient evidence exists about the risk factors for anal sphincter laceration to permit modification of management of labour and delivery to minimize the risk of anal sphincter laceration. Increased awareness of the clinical importance of recognition and repair of anal sphincter laceration may explain the rising incidence.