Laura is 41 years old and has just given birth on the labour ward following an uncomplicated pregnancy. She has one other child born via caesarean section 22 months previously with no complications and she has no medical conditions. Following a successful vaginal delivery, the midwife becomes worried about the extent of bleeding Laura is having and she is unable to deliver the placenta. She can see no obvious cause so presses the emergency call bell. She is tachycardic and hypotensive but her temperature is normal.
Which of these is the most likely cause of her symptoms?