My placenta was low at the 12 week scan as well, so they made sure they checked it at the 20 week scan and it hadn't moved up much, still low (1.5cm from os). It meant that I had to have a couple more scans in the third trimester. Had one at 32 weeks and it hadn't moved much at all. Therefore I had to have a c/s (although I did go into labour spontaneously so my c/s was then classed as emergency).