Three case reports illustrate situations where transvaginal sonography (TVS) can be important for detecting foetal malformation. In each of these cases a reliable diagnosis was not possible by abdominal ultrasound either because the part of the foetus to be examined lay too low in the pelvis or because it could not be imaged clearly enough due to lack of amniotic fluid. As we would recommend in such cases, diagnosis was then made by TVS. Since the area which can be imaged by TVS is limited mechanically by the anatomical situation, however, it should not be used as the primary method for prenatal screening, whereas in certain selected cases it can provide information not obtainable otherwise.