A triage system that prioritizes care according to medical urgency has a favorable effect on safety and efficiency of emergency care. The Dutch obstetric telephone triage system is comparable to physical triage systems. It consists of five urgency levels: resuscitation and life threatening (U1), emergency (U2), urgent (U3), non-urgent (U4) and self-care advice (U5). The purpose of this study was to determine the diagnostic and external validity of the Dutch obstetric telephone triage system in obstetric emergency care. Patients and Methods: The validity of the Dutch obstetric telephone triage system was studied in a prospective observational study in four hospitals. Diagnostic validity of usual care was determined by comparing the assigned urgency level of the Dutch obstetric telephone triage system with a reference standard. This reference standard was obtained by face-to-face clinical assessment in hospital following telephone triage. Clinical follow-up after assessment was also recorded. For statistical analyses, urgency levels were dichotomized into high urgency (U1, U2) and intermediate urgency (U3, U4). Self-care advice (U5) could not be studied because these patients were not referred to hospital. Results: In total, 983 cases (U1-U4) across the four hospitals were included, 625 (64%) cases were categorized as high urgency and 358 (36%) as intermediate urgency. The Dutch obstetric telephone triage system's urgency level agreed with the reference standard in 53% (n=525; 95% CI 50-57%). According to the reference standard the Dutch obstetric telephone triage system had undertriage in 16% (n=160) and overtriage in 30% (n=298) of the cases. Sensitivity for high urgency was 76% (95% CI 72-80), specificity 49% (95% CI 44-53). Positive predictive value and negative predictive value were 60% (95% CI 56-63) and 67% (95% CI 62-72), respectively. After clinical assessment, urgent care was needed in 8.7% (n=31) of the intermediate-urgency cases, none of these cases were life threatening situations. Conclusion: DOTTS shows an acceptable diagnostic validity with room for improvement.