The impact of first birth obstetric anal sphincter injury on the subsequent birth: a population-based linkage study

Abstract

Background With rising obstetric anal sphincter injury (OASI) rates, the number of women at risk of OASI recurrence is in turn increasing. Decisions regarding mode of subsequent birth following an OASI are complex, and depend on a variety of factors. We sought to identify the risk factors for OASI recurrence from first and subsequent births, and to investigate the effect of OASI birth factors on planned caesarean for the second birth. Methods Using two linked population datasets from New South Wales, Australia, we selected women giving birth between 2001 and 2011 with a first birth OASI and a subsequent birth. Multivariable logistic regression was used to identify the association of first and second birth factors with OASI recurrence, and to determine which factors were associated with a planned pre-labour caesarean at the second birth. Results Of 6,380 women with a first birth OASI who proceeded to a subsequent birth, 75.4% had a vaginal second birth, 19.4% a pre-labour caesarean, and 5.2% an intrapartum caesarean. The OASI recurrence rate of 5.7% was significantly higher than the first birth OASI rate of 4.5% (p<0.01). Following adjustment for first and second birth factors, risk factors for recurrence included diabetes at first birth (adjusted odds ratio (aOR) 1.82), and birthweight at second birth ≥4.0kg (aOR 2.34); second birth at 37-38 weeks was associated with decreased odds of OASI (aOR 0.56). First birth factors associated with planned caesarean at second birth included epidural, spinal or general anaesthetic (aOR 1.88); birthweight ≥4.0kg (aOR 1.68); while factors associated with decreased likelihood included Asian country of birth (aOR 0.73), and maternal age< 25 years (aOR 0.81). Conclusions Compared with previous reports, the low OASI recurrence rate (approximately one in twenty) potentially indicates that appropriate decisions are being made about subsequent mode of delivery following first birth OASI. This assertion is supported by evidence of different risk profiles for women who have planned caesareans compared with planned vaginal births.