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

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.

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.

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. Although the OASI recurrence rate of 5.7% was significantly higher than the first birth OASI rate of 4.5% (p < 0.01), this may not reflect a clinically significant increase. Following adjustment for first and second birth factors, first birth diabetes and second birthweight ≥3.5 kg were associated with increased likelihood of OASI recurrence, while first birthweight ≥4.0 kg and second gestation at 37-38 weeks were associated with decreased likelihood. A fourth degree tear at the first birth was the strongest factor associated with planned caesarean at the second birth, with other factors including epidural, spinal or general anaesthetic, birthweight, gestation, country of birth and maternal age.

Compared with previous reports, the low OASI recurrence rate (approximately one in twenty) may reflect appropriate decision-making 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.