Epidemiological studies on adverse effects of prenatal ultrasound—Which are the challenges?

A large number of foetuses are scanned on a routine basis and although it is generally assumed that prenatal ultrasound is safe; very few studies have in fact focused on possible adverse effects in humans. The epidemiological tools when studying possible adverse effects of prenatal ultrasound are randomised controlled trials and observational studies such as cohort and case–control studies. There are advantages and disadvantages with all study designs. In this review, some of the challenges that have to be met are discussed based on experiences from a randomised controlled trial, cohort studies and an ongoing case–control study.