Background
Surgical site infection (SSI) after a caesarean section is of concern (CS) is of concern to both clinicians and women themselves.

Aims
The aim of this study is to identify the cumulative incidence and predictors of SSI in women who are obese and give birth by elective CS.

Materials and Methods
The method used was planned secondary analysis of data from women with a pre‐pregnancy body mass index (BMI) ≥30 kg/m2 giving birth by elective CS in a multicentre randomised controlled trial of a prophylactic closed‐incision negative pressure wound therapy dressing. Data were collected from medical records, direct observations of the surgical site and self‐reported signs and symptoms from October 2015 to December 2019. The Centers for Disease Control and Prevention definition was used to identify SSI. Women were followed up once in hospital just before discharge and then weekly for four weeks after discharge. Blinded outcome assessors determined SSI. After the cumulative incidence of SSI was calculated, multiple variable logistic regression models were used to identify independent risk factors for SSI.

Results
SSI incidence in 1459 women was 8.4% (122/1459). Multiple variable‐adjusted odds ratios (OR) for SSI were BMI ≥40 kg/m2 (OR 1.55, 95% confidence interval (CI) 1.30–1.86) as compared to BMI 30–34.9 0 kg/m2, ≥2 previous pregnancies (OR 1.38, 95% CI 1.00–1.80) as compared to no previous pregnancies and pre‐CS vaginal cleansing (OR 0.55, 95% CI 0.33–0.99).

Conclusions
Our findings may inform preoperative counselling and shared decision‐making regarding planned elective CS for women with pre‐pregnancy BMI ≥30 kg/m2.