ObjectiveTo estimate the population prevalence of anxiety disorders during pregnancy and investigate the diagnostic accuracy of the two-item Generalised Anxiety Disorder scale (GAD-2) for a) GAD and b) any anxiety disorder.DesignCross-sectional survey using a stratified sampling design. Sampling weights were used in the analysis to adjust for the bias introduced by the stratified sampling.SettingInner-city maternity service, South London.Participants545 pregnant women were interviewed after their first antenatal appointment; 528 provided answers on the GAD-2 questions.Main outcome measuresDiagnosis generated by the Structured Clinical Interview for Diagnostic and Statistical Manual of Mental Disorders, 4th edition (SCID).ResultsPopulation prevalence of anxiety disorders was 17% (95% CI 12% to 21%): 5% (95% CI 3% to 6%) for GAD, 4% (95% CI 2% to 6%) for social phobia, 8% (95% CI 5% to 11%) for specific phobia and 2% (95% CI 1% to 4%) for obsessive-compulsive disorder. Post-traumatic stress disorder (PTSD) prevalence was unclear due to higher levels of reluctance to respond to PTSD interview questions but sensitivity analyses suggest population prevalence maybe up to 4% (95% CI 2% to 6%). Weighted sensitivity of GAD-2 for GAD (cut-off ≥3) was 69%, specificity 91%, positive predictive value 26%, negative predictive value 98% and likelihood ratio 7.35. For any anxiety disorder the weighted sensitivity was 26%, specificity 91%, positive predictive value 36%, negative predictive value 87% and likelihood ratio 2.92.ConclusionsAnxiety disorders are common but GAD-2 generates many false positives and may therefore be unhelpful in maternity services.