Assessing quality of obstetric care for low‐risk deliveries; methodological problems in the use of population based mortality data

Background. Studies evaluating safety of different birth settings for low‐risk deliveries are often difficult to interpret because of great methodological problems. Objective. To assess potential bias in comparisons of mortality between maternity institutions with different size and level of care, particularly when using various definitions of low‐risk delivery and when studying stillbirth rates. Design. Population‐based study. Population. The population of 1.74 million births in Norway from 1967 to 1996 recorded in The Medical Birth Registry of Norway. Methods. First we explored the problems of properly identifying low‐risk deliveries from population‐based data and calculated adjusted perinatal mortality rates in sub‐populations by excluding different risk factors. Then we measured the difference in apparent low‐risk deliveries between institutions of different size and level of care. Finally we explored bias by using stillbirths and discuss the loss of statistical power by studying only livebirths. Results. The occurrence of a whole spectrum of risk factors differed between small and large institutions, even after adjustment for birthweight. Although the majority of births were from low‐risk deliveries, only 1/10th of all perinatal deaths occurred in this group after admission to a maternity unit. There was a systematic difference in the reporting of time of death for stillbirths between types of institutions; the rate of stillbirths occurring during delivery was higher among small institutions, while large institutions were more often uncertain in classifying time of death for stillbirths. Conclusions. Adjustments for a large number of different risk factors, large sample‐sizes and caution in including stillbirth as outcome measure are needed when comparisons of safety between different sizes of delivery units are made for low‐risk pregnancies.