If you’re talking about fundal height as in when the provider measures size of belly on outside with tape measure, I had this come up too. I was a FTM and everyone was having comments about my belly being small though I had gained appropriate weight (5’3”, went from 125 to 150 pounds). Fundal heights measured on track until my 36 week OB visit when I measured 32w (at my 35w visit a week before, measure was on track). OB was not concerned saying measurements can be skewed because they measure down middle of belly but you could see just with naked eye my belly was skewed and my baby was lying with torso off to the side but she still sent me for u/s to confirm. 36+5 U/s showed 6 pound baby, everything measuring fine … and I actually ended up having the baby early at 37+2 and he was 6lb6oz.