Objectives: To test the hypothesis that low risk women at the onset of labour with planned home birth have a higher rate of severe acute maternal morbidity than women with planned hospital birth, and to compare the rate of postpartum haemorrhage and manual removal of placenta. Design: Cohort study using a linked dataset. Setting: Information on all cases of severe acute maternal morbidity in the Netherlands collected by the national study into ethnic determinants of maternal morbidity in the netherlands (LEMMoN study), 1 August 2004 to 1 August 2006, merged with data from the Netherlands perinatal register of all births occurring during the same period. Participants: 146 752 low risk women in primary care at the onset of labour. Main outcome measures: Severe acute maternal morbidity (admission to an intensive care unit, eclampsia, blood transfusion of four or more packed cells, and other serious events), postpartum haemorrhage, and manual removal of placenta. Results: Overall, 92 333 (62.9%) women had a planned home birth and 54 419 (37.1%) a planned hospital birth. The rate of severe acute maternal morbidity among planned primary care births was 2.0 per 1000 births. For nulliparous women the rate for planned home versus planned hospital birth was 2.3 versus 3.1 per 1000 births (adjusted odds ratio 0.77, 95% confidence interval 0.56 to 1.06), relative risk reduction 25.7% (95% confidence interval -0.1% to 53.5%), the rate of postpartum haemorrhage was 43.1 versus 43.3 (0.92, 0.85 to 1.00 and 0.5%, -6.8% to 7.9%), and the rate of manual removal of placenta was 29.0 versus 29.8 (0.91, 0.83 to 1.00 and 2.8%, -6.1% to 11.8%). For parous women the rate of severe acute maternal morbidity for planned home versus planned hospital birth was 1.0 versus 2.3 per 1000 births (0.43, 0.29 to 0.63 and 58.3%, 33.2% to 87.5%), the rate of postpartum haemorrhage was 19.6 versus 37.6 (0.50, 0.46 to 0.55 and 47.9%, 41.2% to 54.7%), and the rate of manual removal of placenta was 8.5 versus 19.6 (0.41, 0.36 to 0.47 and 56.9%, 47.9% to ...