Less than 6 lbs in the first trimester is fine. The only thing I'd suggest watching out for is if you fine that you're gaining weight rapidly in the third trimester, particularly the last 8 weeks. Gaining 3+ lbs in a week usually means water weight, and while some edema is normal, too much can be a sign of serious pregnancy complications.
Personal example: I was gaining weight too rapidly my last trimester of pregnancy #1, OB just told me to eat less and move more. I restricted my calories to 1600-1800 a day, but was still gaining as much as 4 lbs in a week. Turns out I had undiagnosed pregnancy-induced heart failure, and almost died two days after my son was born. My OB knew I had edema and maternal hypertension and didn't treat either condition. She ignored the warning signs as "normal symptoms of pregnancy," and it almost cost me my life.