From what I understand, the erosion is due to a high level of oestrogen, which happens when on the pill or when pregnant. It's the softer cells of the cervix that become over-grown. I have read that it looks inflamed and infected.

I do believe it can cause bleeding during pregnancy, and also if it is irritated or 'bumped'.

But thats all I know.

ETA: Thanks BW and Leasha!