Background Childbirth has been linked to postpartum impairment. However, controversy exists regarding the onset and prevalence of post-traumatic stress disorder (PTSD) after childbirth, with seminal studies being limited by methodological issues. This longitudinal prospective study examined the prevalence of PTSD following childbirth in a large sample while controlling for pre-existing PTSD and affective symptomatology. Method Pregnant women in their third trimester were recruited over a 12-month period and interviewed to identify PTSD and anxiety and depressive symptoms during the last trimester of pregnancy, 4-6 weeks postpartum, 12 weeks postpartum and 24 weeks postpartum. Results Of the 1067 women approached, 933 were recruited into the study. In total, 866 (93%) were retained to 4-6 weeks, 826 (89%) were retained to 12 weeks and 776 (83%) were retained to 24 weeks. Results indicated that, uncontrolled, 3.6% of women met PTSD criteria at 4-6 weeks postpartum, 6.3% at 12 weeks postpartum and 5.8% at 24 weeks postpartum. When controlling for PTSD and partial PTSD due to previous traumatic events as well as clinically significant anxiety and depression during pregnancy, PTSD rates were less at 1.2% at 4-6 weeks, 3.1% at 12 weeks and 3.1% at 24 weeks postpartum. Conclusions This is the first study to demonstrate the occurrence of full criteria PTSD resulting from childbirth after controlling for pre-existing PTSD and partial PTSD and clinically significant depression and anxiety in pregnancy. The findings indicate that PTSD can result from a traumatic birth experience, though this is not the normative response.