Events in third stage of labour

The third stage of labour comprises the phase of placental separation; its descent to
the lower segment and finally its expulsion with the membranes. The main events in
the third stage of labour are -
              a) Separation of the placenta
              b) Expulsion of the placenta and membrane

Placental separation -
After the birth of the baby, the uterus measures about 20 cm vertically and 10 cm
antero-posteriorly, the shape becomes discoid. The wall of the upper segment is much
thickened, while the thin and flabby lower segment is thrown into folds. The cavity is
much reduced to accommodate only the after-births. A shearing force is instituted 
between the placenta and the placental site which bring about its ultimate separation.

Separation of the membranes - 
The membranes which are attached loosely in the active part (upper uterine segment)
are thrown into multiple folds. Those attached to the lower segment are already
separated during its stretching. The separation is facilitated partly by uterine contraction 
and mostly by weight of the placenta as it descends down from the active part .

Expulsion of placenta - 
After complete separation of the placenta, it is forced down into the flabby lower uterine
segment or upper part of the vagina by effective contraction and retraction of the uterus.
Thereafter, it is expelled out by either voluntary contraction of abdominal muscles (bearing
down efforts) or by manipulative procedure.