To examine the prevalence and risk factors for blood transfusion during delivery. Design. Register-based retrospective cohort study from Finland. Setting. National Medical Birth Register data during 2006–2008. Sample. A total of 171 731 women having singleton deliveries, of whom 3394 (1.98%) received blood transfusion. Methods. We calculated odds ratios (ORs) with 95% confidence intervals (CIs) by multivariate logistic regression to adjust for confounders related to maternal background and mode of delivery. Main outcome measures. Blood transfusion rates by risk factors. Results. Blood transfusion rate during labor increased slightly, from 1.83% in 2006 to 2.27% in 2008 (p < 0.001), during the study period. The highest rate, almost 4%, was reported in central hospitals. Advanced maternal age and primiparity predisposed to blood transfusion. A previous cesarean section increased these rates also in subsequent vaginal delivery (2.64%) compared with women who had vaginal deliveries only (0.86%, OR 3.14, 95% CI 2.65–3.72). Induction of labor almost doubled the risk for blood transfusion (adjusted OR 1.74, 95% CI 1.60–1.89). All instrumental vaginal deliveries (adjusted OR 2.46, 95% CI 2.25–2.69) and any cesarean sections (adjusted OR 1.80, 95% CI 1.66–1.96) increased this risk. Delivery of a large-for-gestational age newborn increased the blood transfusion risk over twofold. Conclusions. As previous cesarean section includes an increased risk for blood transfusion, even in subsequent deliveries, it is essential to consider the mode of labor carefully. The blood transfusion rate was the highest in central hospitals, suggesting differences in blood transfusion practice.