When can you rule out previa?

[MamaPhD]

We just did an ultrasound at 13 weeks (for nuchal translucency) and everything was fine. Except one thing was that middle of my placenta was totally right on top of birth canal. I know 13 weeks is super early & my uterus will grow so much more later on. But I was wondering around what stage can you start rulling out I will have placenta previa? I have another ultrasound scheduled at 20 weeks for anatomy scan. Will I know by then? How much does placenta actually move over? Thanks
ETA: I know it doesn't actually move over per se but the uterus just grows bigger btw.

[MaerynPearl]

its possible at 20 weeks it may... but then it may not... if it doesnt they will have to schedule you another. At 21 weeks mine was previa but by 27 weeks it was not (they would have followed up with ANOTHER ultrasound at 32 weeks if my 27 week one was still covering)

[MeepyCat]

You can rule out placenta previa just as soon as you see that the placenta is not over the birth canal. How long that takes is highly individual. My experience is that stubborn previas make medical professionals scratch their heads - they don't know why some move and some don't (beyond obvious issues like how far they've got to go). My experience is also that a pregnancy with previa is a pregnancy in which you will have all the ultrasounds there are.

[lillymonster]

I would say it depends. I had it at 20 weeks, went away by 36 weeks

This time I had it at 12 weeks and it was gone by 20 weeks, so I guess it depends on how bad it is and how big you get

[MamaPhD]

Wow so it varies a lot it seems. I'm just sending positive thoughts down to my placenta - scoot over!

[MaerynPearl]

Thats all you can do!

but, according to my doctor they almost always move. So good luck!

[Earthy Mama]

Mine's still previa at 28 weeks-we'll check again in 4 weeks...but it hasn't budged a mm since it was first discovered at 20 weeks.

[karen1968]

Mine was previa at 20 weeks. I haven't had a follow-up scan yet but I have every confidence that it will have moved by then (32 weeks). I am planning a home water birth, so if it doesn't move, I'll be hugely disappointed

From all the threads I've read on here and what I've heard from my midwives, it's common to have previa early on, just because the uterus is so small, but that something like 99% of them move by 36 weeks.