Placenta previa/c section?

So at 20 weeks i had marginal placenta p, at 24 weeks was measuring at 1.4 cm now today at 28 weeks she said I'm at 2.3 cm, which is crazy because about a week ago i had an u/s and they said it was still in the same place, but today told me it had moved almost a whole cm to 2.3 and the ultra sound tech sounded very optimistic about it moving, I'm just wondering where typically docs will let you try for vaginal instead of c section because originally they told me if it didn't move i would absolutely have a cs, but now I'm hoping it's moved enough or is getting much closer to the number i need to try for a vaginal.... Some one with experience please comment