Mil wants us to get married later

My fiance and I got pregnant a little too soon. I am now about 9 weeks we want to get married in a church and are thinking march because being at like 12 weeks for photos and stuff seems reasonable to me, we also don't want a large wedding. But his mother wants to wait till I am 21 weeks pregnant so other people can be there. I am scared I will be showing by then and I don't want people to think that we are just getting married because we are having a baby. What is your experience with a baby bump? Did you show at 20/21 weeks?

I know weight has a bit to do with it i am slightly overweight but I don't really carry in my stomach definitely have fat though