My sister had a low lying placenta when she was pregnant with my nephew and it did move up. At her 20 week ultrasound it was also less than 2cm from her cervix and then at 28 weeks it was more than 4cm. Hopefully it will have moved by your next ultrasound!
if i was you i would call my doctor about the no sex thing bc mine told me that sex could induce labour with the placenta being right there
mine is less then 2 cm from cervix...my doc never told me for no sex.... i have another ultrasound at 26 weeks...lets see. good luck to u.