I had a complete placenta previa that resolved itself by 28 weeks. I was so freaked out at first (I got put on pelvic rest after my 20 week ultrosound and told not to travel outside of the range of medical care just in case) but nothing ever happened and by 28 weeks the placenta had moved completely.
Here's the thread I started at the time if it helps:
https://www.dcurbanmom.com/jforum/posts/list/822627.page