Denhez M, Bouton J M, Engelmann P, Dupray D M
J Gynecol Obstet Biol Reprod (Paris). 1981;10(4):335-47.
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.
超声是目前观察胎盘的最佳手段。因此，有必要了解胎盘发育过程中出现的不同阶段。超声将使我们能够了解这一演变过程，并寻找可能出现的异常情况。胎盘在闭经约第12周时成为一个实体。在妊娠中期可以精确确定胎盘的位置，但胎盘在子宫内的最终位置并不固定，因为不同节段发育不均匀。尽管如此，有必要对妊娠末期胎盘异常植入的风险进行预测。因此，精确确定胎盘位置很有必要。有几个误差来源必须避免：——宫颈内口精确位置的困难。——存在子宫肌层增厚，这为子宫伸展提供储备。这些增厚有时与胎盘位于同一位置，可能难以区分。——胎盘褶皱的存在。有几个不同因素与预后有关：——胎盘下缘相对于宫颈内口的位置。——胎盘上缘相对于子宫底部的位置。从统计学角度看，这是最佳预后指标，因此我们建议进行分类。——最后，胎盘下缘与宫颈内口之间的子宫肌层体积能提供一些信息，并使我们能够了解下段如何伸展。最后，通过在一项研究中综合所有这些因素，才能更好地预测胎盘在子宫内的最终位置。只有在妊娠晚期下段形成后，才能确定胎盘的最终位置，并就患者的治疗方式做出决定。
