At 21weeks without baby bump is not a thing to worry about, u are rather lucky, my kidsis baby bump never showed up until she was 31weeks & above, ppl were all surprised when she had her baby, as for your Anterior placenta is normal same with mine too, then baby gender determination on scan is from 20weeks, though I will advise u wait till 25weeks for more accurate result.
You women are funny. Can you kindly enjoy the pregnancy. At 20 weeks, it is not compulsory to have a bump that early. Some women have a big uterus so baby is going to hide. I started showing like 9months, many people thought I was just about three months. So enjoy the hide and seek. I enjoyed the fact that many people didn't quite know I was pregnant.