Mechanism of separation of placenta

Marked retraction reduces effectively the surface area at the placental
site to aboutits half. But as the placenta is inelastic, it can not keep pace
with such an extent of diminution resulting in its buckling. A shearing force 
is instituted between the placenta and the placental site which bring about 
its ultimate separation. The plane of separation runs through deep spongy
layer of decidua basahs so that a variable thickness of decidua covers the
maternal surface of separated placenta.

There are two way of separation of placenta.

1. Central separation - 

Detachment of placenta from its uterine attachment starts at the centre
resulting in opening up of few uterine sinuses and accumulation of blood
behind the placenta (retroplacental haematoma). With increasing contraction,
more and more detachment occurs facilitated by weight of the placenta and
retroplacental blood until whole of the placenta gets detached.

2. Marginal separation - 

Separation starts at the margin as it is mostly unsupported. With progressive
uterine contraction, more and more areas of the placenta gets separated.
Marginal separation is found mare frequently.

Mechanism of control of bleeding - 
The normal volume of blood flow through the placental site is 500-800 ml/min. 
At the placental separation, this has to be arrested within seconds to prevent
serious haemorrhage. Retraction of the oblique uterine muscle fibres in the
upper uterine segment through which tortuous blood vessels intertwin. The
resultant thickening of muscle exert pressure on the torn vessels action on
clamps thus securing a ligature action.