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ObjectiveTo investigate the association between perineal trauma at childbirth and maternal body mass index (BMI), and estimate the risk of perineal trauma among different BMI groups.

MethodsData were retrospectively assessed from all vaginal deliveries in a UK tertiary maternity unit between 1999 and 2014. Associations between BMI at booking and first- and second-degree tears (minor perineal trauma), third- and fourth-degree tears (obstetric anal sphincter injuries [OASIS]), and frequency of instrumental deliveries were assessed. Multivariate logistic regression included the factors BMI (weight in kilograms divided by the square of height in meters), age, ethnicity, smoking, parity, pregnancy length, episiotomies, instrumental delivery, and birth weight.

ResultsData from 45557 deliveries were used. Compared with women with a normal BMI (<25), odds of minor perineal trauma were significantly reduced among women with obesity (BMI 30 to <35; odds ratio [OR] 0.91, 95% confidence interval [CI] 0.84-0.99) or severe obesity (BMI 35; OR 0.87, 95% CI 0.77-0.98). OASIS was not significantly associated with BMI. Instrumental delivery rates were higher among women with normal BMI (5936/27107; 22.0%) than among those with severe obesity (284/2032; 14.0%).

ConclusionIncreased BMI at booking was associated with a reduced incidence of minor perineal trauma at delivery, but was not associated with OASIS.

Increased body mass index was associated with a reduced incidence of minor perineal trauma.