Often there is no obvious reason for the woman to retain her placenta. There are in some cases though contributing factors associated with a retained placenta, which will also determine how it is treated.
The main circumstances that can lead to a retained placenta are:
The uterus not contracting
A full bladder
'Fiddling' with the uterus
A bicornuate uterus
Adhesion of the placenta
Emotional reactions
The uterus not contracting. If the uterus does not contract the placenta cannot separate. In some cases weaker, uncoordinated contractions can lead to partial separation, not being strong enough to fully detach and deliver the placenta. A strong contraction can be stimulated with either natural interventions or with oxytocic injections.
A full bladder. If the woman's bladder is full the uterus may not be able to contract adequately. The placenta can stay attached or only partially separate, until the bladder is emptied.
'Fiddling' with the uterus. Actions by the caregiver prodding, poking, massaging or 'rubbing up' the uterus before the placenta has separated (often referred to as 'fiddling' with the uterus) can interfere with the complete detachment of the placenta.
These procedures were commonly performed in the past, aimed at stimulating a contraction. We now know that they can often cause weaker, irregular contractions leading to partial separation of the placenta and in some cases retention of the placenta within the uterus. Partial separation can increase the woman's blood loss possibly leading to a
postpartum haemorrhage. Prematurely pulling on the cord before the placenta separates can also cause partial separation.
A bicornuate uterus. This is where the woman has an unusually shaped uterus. A piece of tissue or 'septum' inside the uterus (that was present before the pregnancy) extends from the top of the uterus or fundus, inside the uterus. This is also known as a 'heart-shaped uterus'.