I had the same experience. Goes up by its own. 胎盘距离宫口2cm左右, this is not bad at all. Those that do not go up are at the 宫口, not just close to. Because the 胎盘 does not really move. It is the uterus that grows and stretch to make this 2cm longer and longer. At 20 weeks, the uterus has a long time to grow. You just need to do a follow up ultra sound a few weeks later to confirm it does go up. Relax.