To estimate the prevalence and validate the diagnosis of retained placenta in nulliparous women and the risk of reoccurrence at subsequent vaginal delivery. Design. Nested cohort study. Setting. Department of Gynecology and Obstetrics, university-affiliated teaching hospital. Population. 10 334 nulliparous singleton pregnancies who delivered vaginally at the hospital during 2000–2009. Methods. Data from a computerized database information system were used to identify 287 women who had an ICD-10 diagnosis of retained placenta and 572 randomly selected controls matched by the date of first delivery. At chart review the diagnosis was confirmed by: (1) excessive bleeding <30 minutes after delivery without placental separation, (2) placenta not separated 30 minutes after delivery or (3) confirmation of retained placental tissue >2 hours postpartum. Main outcome measures. Confirmation of the diagnosis and prevalence of retained placenta. Risk of reoccurrence in a subsequent vaginal delivery. Results. The prevalence of retained placenta increased from 2.8 to 7.0% after confirmation according to the set criteria. Of the selected women, 48.4% had a subsequent vaginal delivery. Of these women, 25.3% (23/91) with a previous retained placenta and 5.3% (11/206) without previously retained placenta, experienced retained placenta in subsequent delivery. This corresponds to an adjusted odds ratio of 5.5 (95% confidence interval 2.6–12.7) in the multivariate analysis for recurrence of retained placenta in a subsequent vaginal delivery. Conclusions. The use of the ICD-10 diagnosis of retained placenta underestimated the prevalence. The risk of reoccurrence of retained placenta is significantly increased in a subsequent vaginal delivery.