low lying placenta anyone in the same position

Has anyone got a low lying placenta mine had only moved 1.4cm away from my cervix it's on the back wall my cervix has shortened to 2.3 cant have a stitch put in because they might knick the placenta so been but on rest from work for 4 weeks has this happened to anyone else did the placenta move it's gotta move 5cm away from my cervix to be able to put the stitch in heard that if the placenta is at the back it doesn't tend to move is this true thanks in advance I'm 21 weeks 4 days