Objective. To study whether the selection of low risk parturient women into a separate maternity unit leads to a lower risk of emergency cesarean section, compared to giving birth in a unit with mixed cases. Design. Hospital based registry study. Setting. Maternity units in two university hospitals in Oslo, Norway. Population. All low risk parturient women with attempted vaginal deliveries in the years 2001–2003, a total number of 11,686 deliveries. Methods. Data were obtained from standardized patient records and risks of cesarean section were estimated as odds ratios. Main outcome measures. Emergency cesarean section risk. Results. Compared with women giving birth in a unit with mixed cases, women giving birth in a maternity unit with low risk cases only had a higher risk of emergency cesarean section (OR 1.4; 95% CI 1.2–1.6). Conclusions. Giving birth in a low risk maternity unit is associated with a higher risk of cesarean section for low risk parturient women compared with giving birth in a maternity unit with mixed cases.