Sonography of the low-lying placenta: value of Trendelenburg and traction scans.

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.