Midwife Katie says:
There's a whole host of reasons why this might have happened. A retained placenta occurs in about 2% of all births and statistics show that if it has happened to you before, there's a higher risk it could occur again.
It's a good idea to ask your midwife if she can review the notes from your first labour to root out possible causes. What you can't do much about is a rare condition called placenta accreta, which affects about one in every 2500 births. This is when the placenta gets embedded so abnormally deeply in the wall of the womb, it can't be shed by contractions.
Another common reason is a snapped umbilical cord. This can happen if you're given an injection of syntometrine to make your uterus contract and shed the placenta. If the cord is unusually fragile or the placenta hasn't completely separated, it can snap when the midwife pulls on it. She will then advise you to push out your placenta when you have a contraction. However, sometimes the cervix closes, which means the placenta has to be surgically removed.
Another factor that's easy to avoid is a full bladder in the third stage of labour. The risk of this lessens with your second baby, however, as labour is usually much quicker and the baby's head is less likely to occupy the space in your pelvis for as long, blocking the exit to your bladder.
If a snapped cord was the reason for your retained placenta, or it was anything to do with the way your labour was handled, talk to your midwife. By allowing the placenta to expel naturally, you may be able to avoid the cervix closing prematurely.