• Anal sphincter injury;
  • episiotomy;
  • vaginal birth

Objective  Mediolateral episiotomy is associated with lower rates of significant perineal tears than midline episiotomy. However, the relationship between precise angle of episiotomy from the perineal midline and risk of third-degree tear has not been established. This study quantifies this relationship.

Design  Case–control study.

Setting  National Maternity Hospital, Dublin, Ireland.

Sample  One hundred primiparous women who had undergone right mediolateral episiotomy 3 months previously.

Methods  Two groups of primiparous women were compared. Cases had sustained clinically apparent anal sphincter injury during delivery, while controls had not. The angle of episiotomy measured from the midline was marked on a superimposed sheet of transparent plastic film and measured using a protractor. Data were analysed using Student's t test, chi-square test and logistic regression analysis.

Main outcome measures  Angle of mediolateral episiotomy from the perineal midline.

Results  Fifty-four cases and 46 controls were assessed. Cases were more likely to have undergone assisted delivery and consequently to have been delivered by an obstetrician than by a midwife. The mean angle of episiotomy measured significantly smaller in cases (30°, 95% CI 28–32°) than in controls (38°, 95% CI 35–41°; P < 0.001). Analysis showed a 50% relative reduction in risk of sustaining third-degree tear for every 6° away from the perineal midline that an episiotomy was cut.

Conclusions  These results show that a larger angle of episiotomy is associated with a lower risk of third-degree tear and mediolateral episiotomy incisions should be made at as large an angle as possible to minimise the risk of sphincter disruption.