BACKGROUND: Obstetric anal sphincter injuries (OASIs) are associated with maternal morbidity; however, it is uncertain whether gestational diabetes (GDM) is an independent risk factor when considering birthweight mode of birth and episiotomy. AIMS: To compare rates of OASIs between women with GDM and women without GDM by mode of birth and birthweight. To investigate the association between episiotomy, mode of birth and the risk of OASIs. METHODS: A population-based cohort study of women who gave birth vaginally in NSW, from 2007 to 2013. Rates of OASIs were compared between women with and without GDM, stratified by mode of birth, birthweight and a multi-categorical variable of mode of birth and episiotomy. Adjusted odds ratios (aOR) and 95% confidence intervals (CI) were calculated by multivariable logistic regression. RESULTS: The rate of OASIs was 3.6% (95% CI: 2.6-2.7) vs 2.6% (95% CI: 3.4-2.8; P < 0.001) among women with and without GDM, respectively. Women with GDM and a macrosomic baby (birthweight >/= 4000 g) had a higher risk of OASIs with forceps (aOR 1.76, 95% CI: 1.08-2.86, P = 0.02) or vacuum (aOR 1.89, 95% CI: 1.17-3.04, P = 0.01), compared with those without GDM. For primiparous women with GDM and all women without GDM, an episiotomy with forceps was associated with lower odds of OASIs than forceps only (primiparous GDM, forceps-episiotomy aOR 2.49, 95% CI: 2.00-3.11, forceps aOR 5.30, 95% CI: 3.72-7.54), (primiparous without GDM, forceps-episiotomy aOR 2.71, 95% CI: 2.55-2.89, forceps aOR 5.95, 95% CI: 5.41-6.55) and (multiparous without GDM, forceps-episiotomy aOR 3.75, 95% CI: 3.12-4.50, forceps aOR 6.20, 95% CI: 4.96-7.74). CONCLUSION: Women with GDM and a macrosomic baby should be counselled about the increased risk of OASIs with both vacuum and forceps. With forceps birth, this risk can be partially mitigated by performing a concomitant episiotomy.