Obstetric anal sphincter injury rates among primiparous women with different modes of vaginal delivery.
OBJECTIVE
To determine whether rates of obstetric anal sphincter injuries (OASIS) are continuing to increase and whether risk of OASIS according to mode of delivery is constant over time.


METHODS
In a retrospective population-based study, data were obtained for vaginal singleton vertex deliveries at 37-41 weeks of pregnancy among primiparous women in New South Wales, Australia, between January 2001 and December 2011. Annual OASIS rates were determined among non-instrumental, forceps, and vacuum deliveries with and without episiotomy. Multivariable logistic regression was used to determine adjusted odds ratios for each delivery mode category by year. Trends in adjusted odds ratios over time for each delivery category were compared.


RESULTS
OASIS occurred in 955 (4.1%) of 23 081 deliveries in 2001 and 1487 (5.9%) of 25 081 deliveries in 2011. After adjustment for known risk factors, the only delivery categories to show statistically significant increases in OASIS over the study period were non-instrumental deliveries without episiotomy (linear trend P<0.001) and forceps deliveries with episiotomy (linear trend P=0.004).


CONCLUSIONS
Overall, OASIS rates have continued to increase. Known risk factors do not fully explain the increase in OASIS rates in non-instrumental deliveries without an episiotomy and in forceps deliveries with an episiotomy.
Clinical and Population Perinatal Health Research, The Kolling Institute, Northern Clinical School, University of Sydney, St Leonards, NSW, Australia. Electronic address: amanda.ampt@sydney.edu.au.
Amanda J
AJ
Ampt
Clinical and Population Perinatal Health Research, The Kolling Institute, Northern Clinical School, University of Sydney, St Leonards, NSW, Australia.
Jillian A
JA
Patterson
Clinical and Population Perinatal Health Research, The Kolling Institute, Northern Clinical School, University of Sydney, St Leonards, NSW, Australia.
Christine L
CL
Roberts
Clinical and Population Perinatal Health Research, The Kolling Institute, Northern Clinical School, University of Sydney, St Leonards, NSW, Australia.
Jane B
JB
Ford
D000328:Adult; D001003:Anal Canal / Q000293:injuries*; D036861:Delivery, Obstetric / Q000009:adverse effects* / Q000379:methods; D004841:Episiotomy / Q000009:adverse effects / Q000706:statistics & numerical data; D005113:Extraction, Obstetrical / Q000009:adverse effects / Q000706:statistics & numerical data; D005260:Female; D006801:Humans; D016015:Logistic Models; D009517:New South Wales / Q000453:epidemiology; D007744:Obstetric Labor Complications / Q000453:epidemiology*; D009772:Obstetrical Forceps; D011247:Pregnancy; D012189:Retrospective Studies; D012307:Risk Factors; D014620:Vacuum Extraction, Obstetrical / Q000009:adverse effects / Q000706:statistics & numerical data; D055815:Young Adult
Forceps; International Classification of Diseases; Obstetric anal sphincter injury; Record linkage study; Vacuum