OBJECTIVE: To analyse trends in the number of ultrasound examinations in relation to the effectiveness of prenatal detection of birth defects using population-based data in France. DESIGN: A multiple registry-based study of time trends in resource use (number of ultrasounds) and effectiveness (proportion of cases prenatally diagnosed). SETTING: Three registries of congenital anomalies and claims data on ultrasounds for all pregnant women in France. PARTICIPANTS: There were two samples of pregnant women. Effectiveness was assessed using data from three French birth defect registries. Resource use for ultrasound screening was based on the French national healthcare database. MAIN OUTCOME MEASURES: The main outcome measures were prenatal diagnosis (effectiveness) and the average number of ultrasounds (resource use). Statistical analyses included linear and logistic regression models to assess trends in resource use and effectiveness of prenatal testing, respectively. RESULTS: The average number of ultrasound examinations per pregnancy significantly increased over the study period, from 2.47 in 2006 to 2.98 in 2014 (p=0.005). However, there was no significant increase in the odds of prenatal diagnosis. The probability of prenatal diagnosis was substantially higher for cases associated with a chromosomal anomaly (91.2%) than those without (51.8%). However, there was no evidence of an increase in prenatal detection of either over time. CONCLUSIONS: The average number of ultrasound examinations per pregnancy increased over time, whereas the probability of prenatal diagnosis of congenital anomalies did not. Hence, there is a need to implement policies such as high-quality training programmes which can improve the efficiency of ultrasound examinations for prenatal detection of congenital anomalies.