V. The Radiological Localisation of the Placenta

Abstract

During the last 5 years I have been localising the placenta by means of antero-posterior and lateral radiographs with the patient so placed that the foetus itself could be used as a contrast medium. Interpretation of the radiographs requires a knowledge of certain elementary physical and anatomical factors: The placenta lies between the uterine wall and the liquor amnii, all of which have the same radiographic density (Moir, 1944). Although the placenta cannot be differentiated from these surrounding tissues, its situation is usually revealed by a crescentic-shaped thickening of the uterine wall. It can be shown experimentally that this “placental shadow” is not due to liquor, by taking radiographs in various positions (Reid, 1949b). The foetus (sp. gr. 1055–1058) being relatively heavier than the liquor amnii (sp. gr. 1008–1009) tends to seek the lowest part of the amniotic sac, which itself varies with the position of the patient. The forces acting on the foetus when the mother is standing erect can be resolved into vertical and horizontal components, due to gravity and the anterior abdominal muscles respectively: the resultant causes the presenting part to be directed towards the pelvic inlet. The horizontal force is exerted directly backwards and also in a postero-lateral direction from either side; the latter part of this component and the effect of gravity are responsible for the appearances in the erect or semi-erect anteroposterior radiograph, where the presenting part is normally seen lying approximately in the central axis of the inlet.