Taurean, just came across this thread. I also had a low lying placenta at 20 weeks. Mine was 1.5cm (I think). At 32 weeks it had moved back to 3cm. We were going to do another scan at around 37/38 weeks to check again but I went into labour at 36+6 and DS was born early via c/s. My OB likes placentas to be at least 5cm away from the o/s to have a VB, other OB's are happy for it to be a bit closer, so it depends on your OB I guess. Turns out my placenta had an extra bit on top of it too, so it was bigger than normal as well.

If you're really, really keen on a VB definitely push for scans toward the end of your pregnancy and go for the internal ones too because they can get a better measurement.

Good luck, keep us updated