Retained placenta can be broadly divided into:
- failed separation due to failure of uterine contraction or morbid adherence
- separated but retained due to e.g. a snapped cord contracted uterus, closed cervix, …
Placenta accreta is the morbid adherence of the placenta to the uterus, and it is a very rare condition.
There are three main causes of retained placenta:
- uterine atony – the uterus stops contracting
- trapped placenta – behind a closed cervix
- placenta accreta – an area of the placenta remains attached because it is deeply embedded into the uterus wall