OBJECTIVES: To assess whether the efficacy of screening for fetal malformations is affected by patient-, staff-, team- or work environment-related factors. METHODS: A prospective cohort study was carried out at a Danish university hospital with 4000 deliveries per year. In total, 7963 fetuses were examined by midtrimester ultrasound scan during 1997 and 1998. The infants were followed up to the age of at least 1 year. The main outcome measures were the detection rate (DR) of fetal abnormalities in relation to patient factors, staff factors, team and work environment factors. RESULTS: Significant malformations were detected in 100 fetuses or infants, corresponding to an incidence of 1.3%. The prenatal DR was 60%. High maternal body mass index (BMI) was associated with a lower DR, while the presence of twins, the gestational age at time of screening and the sonographer's level of experience were not. Fetuses with a prenatally undetected malformation were more often examined by more than one sonographer than fetuses without congenital malformations. Workload or monotony did not seem to affect the efficacy of screening. There was a trend towards a lower detection rate at midday and when most of the staff were at work. CONCLUSIONS: Some team and work environment factors that may affect the detection rate of fetal abnormalities were identified and have been changed: appointments for screening have been reorganized, regular breaks for all the staff introduced and second opinions are given by a specialist in fetomaternal medicine. The new system will be followed up to ensure that the reorganization has not created new organizational mistakes.