Objective: To assess whether the risk of severe blood loss is increased in semi-sitting and sitting position, and if so, to which extent blood loss from perineal damage is responsible for this finding. Design: Secondary analysis of data from a large trial. Setting: Primary care midwifery practices in the Netherlands. Population: About 1646 low-risk women who had a spontaneous vaginal delivery. Methods: Blood loss was measured using a weighing scale and measuring jug. Logistic regression analysis was used to examine the net effects of birthing position and perineal damage on blood loss greater than 500 ml. Main outcome measures: Mean total blood loss and incidence of blood loss greater than 500 ml and 1000 ml. Results: Mean total blood loss and the incidence of blood loss greater than 500 ml and 1000 ml were increased in semi-sitting and sitting position. In logistic regression analysis, the interaction between birthing position and perineal damage was almost significantly associated with an increased risk of blood loss greater than 500 ml. Semi-sitting and sitting position were only significant risk factors among women with perineal damage (OR 1.30, 95% CI 1.00-1.69 and OR 2.25, 95% CI 1.37-3.71, respectively). Among women with intact perineum, no association was found. Conclusions: Semi-sitting and sitting birthing positions only lead to increased blood loss among women with perineal damage. © RCOG 2007 BJOG An International Journal of Obstetrics and Gynaecology.