OBJECTIVES: The purpose of this study was to determine the relative risk of deep vein thrombosis (DVT) during pregnancy and puerperium in a setting that was not influenced by diagnostic suspicion and referral bias. STUDY DESIGN: Three hundred ninety-three consecutive females with suspicion of a first episode of venous thrombosis were referred in the same way to diagnostic centers. Medical history was obtained before objective testing for deep vein thrombosis. RESULTS: Seventy-six of the 393 females were diagnosed as having deep vein thrombosis; the others were used as control subjects. The odds ratios for pregnancy, cesarean section, and puerperium after adjustment for age, center, family history, and calendar time were, respectively, 5.7 (95% CI 2.5-12.9), 4.0 (95% CI 1.3-12.6), and 2.9 (95% CI 1.2-6.9). CONCLUSION: In this case-control study, in which patients and control patients were subject to the same referral and diagnostic procedures, we found similar risk estimates for pregnancy and puerperium as in previous studies. Our findings show that previous studies were less affected by diagnostic suspicion and referral bias than presumed.