Hey hun,
I was diagnosed with same thing at 18 weeks, I have already done 3 u/s and im 28 weeks now, they ended up finding out I have marginal previa(low laying placenta with part on os/cervix.) usually they dont worry unless there is bleeding/pain, I was spotting a bit and I got bedrest and I was just in pain everyday.
If it doesnt move up by 36 week u/s I will have to have c-section and I have no choice with low laying you can have a natural birth, I did alot of research as I like to know what is going with my body.
As long as placenta isnt covering any part of the of the cervix you wont need a c-section and your still early that it can move up as the same can happen to me if you want anymore info send me an email im always weilling to help especially since your in the same sitiuation.
Take care and wish you lots of luck!!
Jess