I'm 18 weeks and have gained 10lbs. lol But I gained them by the time I was 14-15 weeks, you'll be alright. :) My Mom gained a lot of weight with me, too.
5 lbs is good. I gained 50 with my daughter, and am 33 weeks and gained exactly 40lbs at this point, which is considered alot alot. But everyone is different because their body types are different. As long as you eat healthy you are fine