Hokiemom, I am really surprised that a 4-6lb weight gain at 16 weeks is 1lb over
that doesn't seem like much at all! Especially if you were healthy weight to begin with
I was slim prepreg too, bmi - 21. I was last weighed at 20w and I am up 6lb. I have mixed feelings about it... that calculator you linked told me I'm 2lb under the low range
EFF. I am hungry often and eat whenever I feel the pang (not kidding - 430am, whatever, I feed the hunger!). Plus I eat a ton of pasta and noodles (whole wheat)... but I did begin exercising and doing yoga recently too. I will see my doc in 1.5 weeks and see if she is concerned.
I honestly think you are fine. This is the calculator I have been using - see if it gives you a different result
https://www.babycenter.com/pregnancy-weight-gain-estimator