Objective: Dynamic ultrasonographic imaging of the third stage of labor was performed to document ultrasonographic findings and to present new perspectives into third-stage mechanisms.
Study design: Twenty-five normal deliveries and five with prolonged third-stage labor were studied.
Results: 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.
Conclusion: 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.