There is evidence that 3.17% of women report post-traumatic stress disorder (PTSD) after childbirth. This meta-analysis synthesizes research on vulnerability and risk factors for birth-related PTSD and refines a diathesis–stress model of its aetiology. Systematic searches were carried out on PsycINFO, PubMed, Scopus and Web of Science using PTSD terms crossed with childbirth terms. Studies were included if they reported primary research that examined factors associated with birth-related PTSD measured at least 1 month after birth. In all, 50 studies (n = 21 429) from 15 countries fulfilled inclusion criteria. Pre-birth vulnerability factors most strongly associated with PTSD were depression in pregnancy (r = 0.51), fear of childbirth (r = 0.41), poor health or complications in pregnancy (r = 0.38), and a history of PTSD (r = 0.39) and counselling for pregnancy or birth (r = 0.32). Risk factors in birth most strongly associated with PTSD were negative subjective birth experiences (r = 0.59), having an operative birth (assisted vaginal or caesarean, r = 0.48), lack of support (r = −0.38) and dissociation (r = 0.32). After birth, PTSD was associated with poor coping and stress (r = 0.30), and was highly co-morbid with depression (r = 0.60). Moderator analyses showed that the effect of poor health or complications in pregnancy was more apparent in high-risk samples. The results of this meta-analysis are used to update a diathesis–stress model of the aetiology of postpartum PTSD and can be used to inform screening, prevention and intervention in maternity care.