My girlfriend had a lower lying placenta which showed up on her 20 week scan. We were told that she would have to have extra scan to keep an eye on it. The doctors said the placenta would move up towards the end of the pregnancy in most cases but hers didnt. In the end she was told that she had a condition called placenta previa which means the placenta in completely covering the birth canal. She started to have a bleed at 36 weeks because of this. Our baby was born by emergency c section.

I think placenta is not that common and that in most cases the placenta ends up being where it should with presenting problems