Aim: To examine risk of neonatal death after low risk pregnancies in relation to size of delivery units.
Methods: A population based study of live born singleton infants in Norway with birthweights of at least 2500 g was carried out. Antenatal risk factors were adjusted for.
Results: From 1972 to 1995, 1.25 million births fulfilled the criteria. The neonatal death rate was lowest for maternity units with 2001-3000 annual births and steadily increased with decreasing size of the maternity unit to around twice that for units with less than 100 births a year (odds ratio 2.1; 95 % confidence interval 1.6 to 2.8). Institutions with more than 3000 deliveries a year also had a higher rate (odds ratio 1.7; 95% CI 1.4 to 2.0), but analyses suggest that this rate is overestimated.
Conclusion: Around 2000 to 3000 annual births are needed to reduce the risk of neonatal deaths after low risk deliveries.