Dynamic ultrasonographic imaging of the third stage of labor: new perspectives into third-stage mechanisms

Dynamic ultrasonographic imaging of the third stage of labor was performed to document ultrasonographic findings and to present new perspectives into third-stage mechanisms.

Twenty-five normal deliveries and five with prolonged third-stage labor were studied.

Normal third-stage labor could be divided into four phases: (1) latent phase, characterized by thick, placenta-free wall and thin, placenta-site wall; (2) contraction phase, with thickening of placenta-site wall (from < 1 cm to > 2 cm); (3) detachment phase, in which the placenta completes its separation and detaches; and (4) expulsion phase, with a sliding movement of the placenta. Although oxytocic agents were routinely used, they do not seem to influence the findings. In five cases with retained placenta the placenta-site wall was initially thin. In four of them it became thick, and the placenta was removed by traction of the cord, whereas in the fifth case the placenta-site wall remained thin and the placenta had to be removed manually.

Shearing forces seem to tear the decidual septae and thereby separate the placenta. This process is completed only when the placenta-site wall attains full thickness. In cases of prolonged third-stage labor, traction of the cord should be applied only when this phase is completed and the actual sliding movement of the placenta is observed.