if you have recently had a baby ....the entire placenta cannot have been retained..had it not been delivered after the birth..the midiwfe would have taken steps to remove it !
if you have recently had a baby and are now experiencing :
heavy blood loss ( heavier than it has been!)
temperature
feverishness
passing clots
feeling generally unwell...
that is an indication that tiny parts of the placenta have been left behind.they will come out as the blood is lost. you might need to see the doctor and may require antibiotics
hope that helps!