low lying placenta? can it actually go the wrong way instead of moving out of the way?

[is it puppies?]

I had my first (maybe only) ultrasound this week at 18w5d and it turns out my placenta is low. It's low-posterior my MW says that it's about 2cm away from the opening and she's not worried at all and says it's up to me if I want another US (which is recommended by the ultrasound lab).

Everything I've read is quite reassuring about the placenta moving up as the uterus stretches but I can't find any info that talks about whether or not it can actually go the wrong way or perhaps grow over my cervix as the placenta gets bigger, instead of just moving out of the way?

Should I think about booking a second US? am I just being paranoid lol?

(I know I have that tendency)

Thanks

[parsley]

Hugs. I know this isn't good news but try not to worry.

From my experience and reading (and I have read just about everything there is to read on the subject), I do not think it is possible to have the placenta go from low lying to previa.  I had low-lying with my first and complete placenta previa (at term) with my second.  No one and nothing ever suggested that if it cleared it could move again.  The uterus stretches upward as the pregnancy progresses and that stretching pulls the placenta up and out of the way. 

[MeepyCat]

No, the placenta can't get closer to the cervix. 

As your uterus grows, all the points within it get further apart.  They don't get closer together. 

Sometimes, ultrasound techs are wrong or scans are badly read, so a second ultrasound is a good idea.  But it's an "in case the first one was wrong or missed something" good idea, not "the placenta might creep closer to the cervix again".

[parsley]

I think MeepyCat is right in her interpretation-- follow up u/s is just to make sure the reading of the first one is right.

[Chrisa]

Actually 2cm from the cervix sounds like a safe, normal distance to me. Why did she classify it as low-lying, how far do they want the edge to be from the cervix? There isn't that much space in there that early!