20 weeks pregnant and haven't gained any weight?

I haven't gained any weight at all through out my pregnancy, I don't have morning sickness and have a very healthy appetite. Is this normal not to be gaining weight? My ob hasn't said anything about it but he is a man of few words and also he has never listened to the babies heartbeat through a dropper or even a stethoscope is this normal? With my last pregnancy (different ob) he listened to the baby every appointment and also felt my tummy for growth. I haven't had any of that with this dr, only weight checks, blood pressure checks and urine checks. Is this ok?