OBJECTIVE: for some women childbirth is physically and psychologically traumatic and meets Criterion A1 (threat) and A2 (intense emotional response) for Posttraumatic Stress Disorder of the Diagnostic and Statistical Manual of Mental Disorders Fourth Edition (DSM-IV).This study differentiates Criterion A1 and A2 to explore their individual relationship to prevalence rates for posttraumatic stress, each other, and associated factors for childbirth trauma. DESIGN AND SETTING: women were recruited at three hospitals from October 2008 to October 2009. Questionnaires were completed at recruitment and at 14 days post partum. PARTICIPANTS: women in the third trimester of pregnancy (n=890) were recruited by a research midwife while waiting for their antenatal clinic appointment. Participants were over 17 years of age, expected to give birth to a live infant, not undergoing psychological treatment, and able to complete questionnaires in English. FINDINGS: this study found 14.3% of women met criteria for a traumatic childbirth. When the condition of A2 was removed, the prevalence rate doubled to 29.4%. Approximately half the women who perceived threat in childbirth did not have an intense negative emotional response. Predictors of finding childbirth traumatic were pre-existing psychiatric morbidity, being a first time mother and experiencing an emergency caesarean section. KEY CONCLUSIONS: the fear response is an important diagnostic criterion for assessing psychologically traumatic childbirth. The identification of risk factors may inform maternity service delivery to prevent traumatic birth and postpartum approaches to care to address long-term negative consequences. IMPLICATIONS FOR PRACTICE: prevention and treatment of traumatic childbirth are improved through knowledge of potential risk factors and understanding the woman's subjective experience.