Ultrasound exposure time in routine obstetric scanning

The duration of exposure to ultrasound was measured in 1274 patients having a routine examination during their first visit to an antenatal clinic. The measured median exposure time was 105 s. Since pulsed ultrasound was used (duty cycle of 1 in 800) the actual median maternal exposure time was 131 ms. Exposure of the fetus would usually be less since it rarely occupies the full width of the image. The relevance of exposure time to the safety of diagnostic ultrasound and the implications for users and manufacturers are discussed.