Do you have to put on weight for your baby to be healthy?

I have been confused with my weight and I was wondering if anyone else had this problem as well. I was on the Depo shot for a year and a half and off of it for 3 years and of which the whole time I never had a period or spotted. I gained 30 ibs on the shot so when I lost 20 over the summer I was excited thought I was doing good. Well turns out i was 18 weeks pregnant with a pre pregnancy weight of 169. Ok so anyways to the point I have only gained about 6 pounds my whole pregnancy and I keep getting told that your suppose to gain so much weight for your baby to be healthy and all but I can't keep my weight on or even a steady weight anymore. I'm a first time mom and just want to know if my little boy will be ok if I can't keep on my weight.