Pathological separation of the placenta from its uterine attachment results in a maternal bleed from the opened sinuses.
Initially, the decidua basalis splits, leaving a thin layer adherent to the myometrium. A decidual haematoma is formed with destruction of adjacent placenta.
There is a spectrum of presentation:
- concealed - no actual bleeding is seen - it remains in the uterus
- revealed - blood tracks between the membranes and the uterus and escapes vaginally
- mixed - most usual