A little back story.....

At my 21 week ultrasound my son was measuring in the 61st percentile. At my 32 week appointment I expressed concern that my belly didn’t look like it was growing. The doctor agreed and did a growth ultrasound. At 32 weeks he was measuring in the 19th percentile. His head is in the 50th and his abdomen is in the 10th. That jump is huge. The doctor made it seem like that completely normal. But that doesn’t sound right to me. I’m now 34 weeks and my belly still looks like I’m in my second trimester. I’m still feeling kicks so that’s good but I had a family have this happen to her and she ended up giving birth to a stillborn. I’m a first time mom so they haven’t taken any of my concerns seriously this whole time. I have uterine prolapse that they decided to ignore until I was extremely persistent. In this situation I have no clue what to even ask them to do. The percentile change is too drastic of a change for me to ignore. How should I go about talking to the doctors so that they respect me and actually listen to my concerns? Any advice?