Placental abruption is essentially a clinical diagnosis, determined
by the features described above and confirmed by the demonstration
after delivery of a retroplacental clot indenting the placental
substance. Ultrasound imaging has a much smaller role than in
the diagnosis of placenta praevia. In acute severe abruption, the
ultrasound appearances are often unimpressive because the fresh
retroplacental clot has acoustic characteristics which may be very
similar to those of the placenta itself. In less severe cases in which
the pregnancy continues, the clot becomes increasingly echo-free
with time, and therefore more obvious to the ultrasonographer
(Nyberg 1987).