This is sort of confusing at first, but here’s how it works:
On the left hand side is “number of weeks.” Find out where you were with your most recent ultrasound.
Across the bottom is the “distance from or over the os.” (The os is your cervical opening.) You may not have been given this number. If not, call your OB and ask how many millimeters your previa is from (or over) your cervix. (They may also give it to you in centimeters, just convert it by moving the decimal – 1.5 cm = 15 mm).
Find where those two numbers intersect on this image. The closer it is to the blue zone, the better the odds of your previa resolving. The closer it is to the red zone, the greater your odds of your previa persisting.
Another study I found indicated that second trimester previas that were less than 1.5 cm over the os were likely to resolve; previas that were 2.5 cm or more over the os were likely to persist.