Jeffrey R B, Laing F C
AJR Am J Roentgenol. 1981 Sep;137(3):547-9. doi: 10.2214/ajr.137.3.547.
In later stages of pregnancy, acoustic shadowing from a fetal head may obscure the termination of a low-lying placenta and prevent sonographic imaging of the internal cervical os. The evaluation of a marginal or partial placenta previa in this setting may be difficult. Placing the mother in Trendelenburg position and applying gentle upward traction on the fetal head allows amniotic fluid to cover the lower uterine segments. This provides an acoustic window to visualize the relation of the termination of the placenta to the internal os. This technique was evaluated prospectively in a series of 217 patients and was successful in 49 of 50 patients in whom conventional studies failed to image the termination of the placenta or the region of the internal os.
在妊娠后期，胎儿头部的声影可能会遮挡低置胎盘的下缘，从而妨碍对宫颈内口的超声成像。在这种情况下，评估边缘性或部分性前置胎盘可能会很困难。将母亲置于头低脚高位，并对胎儿头部施加轻柔的向上牵引，可使羊水覆盖子宫下段。这提供了一个声学窗口，以观察胎盘下缘与宫颈内口的关系。这项技术在217例患者中进行了前瞻性评估，在50例传统检查未能显示胎盘下缘或宫颈内口区域的患者中，有49例获得成功。
