[Placental localisation : screening and prognosis (author's transl)]

Ultrasounds are the best means at present available for observing the placenta. It follows that it is necessary to appreciate the different phases that occur in placental development. Ultrasound will allow us to work out this evolution and to look for abnormalities that may develop. The placenta becomes an entity at about the 12th week of amenorrhoea. Precise localisation of the placenta is possible in the second trimester of pregnancy, but the final site of the placenta is not fixed in the uterus because different segments develop unevenly. All the same, it is necessary to make a prognosis of the risks of an abnormal insertion of the placenta at the end of pregnancy. It is therefore necessary to be precise in localising the placenta. There are several sources of error which have to be avoided: -- difficulty of exact localisation of the internal os of the cervix. -- the existence of thickening of the myometrium, which forms a reserve for uterine stretch.. These thickenings sometimes come at the same places as the placenta and it may be difficult to differentiate one from the other. -- the presence of placental folds. There are several different elements involving prognosis : -- the site of the lower edge of the placenta in relationship to the internal os. -- the site of the upper edge of the placenta in relationship to the fundus of the uterus. This, statistically, is the best index of prognosis, and so we suggest a classification. -- finally the volume of the myometrium between the lower edge of the placenta and the internal os gives an idea and allows us to work out how the lower segment can stretch. Finally, it is by combining all these elements in a study that a better prognosis can be worked out for the final site of the placenta in the uterus. It is only in the third trimester of pregnancy after the lower segment has formed that the final site of the placenta can be confirmed and that a decision can be taken on the way patient should be treated.