Hey Girl,
Sorry I can't be too much help, but my sis was 38 weeks and the placenta was over the cervix. They booked in a CS then she had another scan just to be sure and it had moved out of the way. I am not sure how many centimeters, but it can't have been much given that it was over it just the week before. Maybe Google it to see what it means - placental preavia (sp?) it is called - to see how many cms are recommended.

Good luck hun