Background Childbirth-related post-traumatic stress disorder (CB-PTSD) is an underrecognized condition with consequences for mothers and infants. This study aimed to determine risk factors for CB-PTSD symptoms across countries within a stress-diathesis framework, focusing on antenatal, birth-related, and postpartum predictors. Methods The INTERSECT cross-sectional survey (April 2021-January 2024) included 11,302 women at 6-12 weeks postpartum. The study was carried out across maternity services in 31 countries. Outcomes were CB-PTSD diagnosis, symptom severity, and perceived traumatic birth, assessed with the City Birth Trauma Scale. Multiple risk factors were assessed, including preexisting vulnerability, pregnancy, birth, and infant-related factors. All models were adjusted for country-level variation as a random effect. Results Models explained substantial variance across all outcomes (conditional R² = 0.53-0.58). Negative birth experience was the strongest predictor (e.g. odds ratio [OR] = 0.82, 95% confidence interval [CI] = 0.80-0.84 for diagnosis). Ongoing maternal complications predicted both CB-PTSD diagnosis and symptoms (e.g. OR = 1.61, 95% CI = 1.41-1.84), and major infant complications were associated with CB-PTSD diagnosis (OR = 1.63, 95% CI = 1.29-2.07). Reports of perceived danger to self or infant (criterion A) were linked to higher CB-PTSD symptoms and traumatic birth ratings (e.g., β =0.25, 95% CI = 0.21-0.29). Other predictors reached significance but showed small effects. Conclusions Findings support a stress-diathesis framework, showing that while pre-existing vulnerabilities contribute, birth-related stressors exert the strongest influence. Trauma-informed maternity care should prioritize these factors, with attention to women's appraisals of birth.