All my 4 pregnancies have low-lying placentas & dey moved up by 3rd trimester in my 1st 3 pregnancies.
As for my 4th pregnancy, the placenta didn't move up by 28wks scan & gynae scheduled me for more frequent scans as I neared my due date. Mine is partial placenta previa, meaning the placenta is is very near or covers part of the cervical opening. If I can recall correctly, if the placenta is less than 2cm away from cervix, I won't be allowed to have a normal delivery. Luckily mine was 3.5cm away from cervix at my 39wk scan. So I was able to go for normal delivery