Don't worry about the weight gain. With DS, I gained nothing til about 18w, then gained 18lbs until 38w, then lost 2 lbs before delivering a 9lbr 4 days past my EDD.
This time around, I have gained 6lbs in the first 13w.
Congrats on hearing the hb. My DH was pretty excited to hear it for the first time too. I think it makes things much more real for dad's to hear the hb. Mom's get to expience all the changes in their bodies and feel movement, etc. Dad's have to go on what we tell them and it isn't as real til they can see/hear/feel for themselves.