How long before I see muscle gains on the scale? Please help!

I started working out with my personal trainer 4 months ago and weighed 110lbs with a body fat percentage of 19%. I maintained 110 up until about 3 weeks ago when my scale started showing me 112. I only weigh myself once a week at the exact same time and I'm aware of the normal body fluctuations, but the 112 has been pretty consistent for 3 weeks now. It is a brand new scale probably only a month old and it takes my body fat percentage as well. And with the new 112 my body fat is now between 17.8-18.2%. (I'm not concerned about the body fat % but thought I should include it). I train with my trainer 2-3 times a week and have seen significant changes in my body. It looks a lot better since training-- my legs (which have always been short, fat and stalky) have toned up and I can see the muscle poking through and my arms have developed some nice definition. My question is: can I attribute the 2lb increase to muscle? I hope I won't get yelled at for this...but is it practical to assume that I could have gained 2 lbs of muscle in a 4 month period?

Yeah provided your nutrition is on the right track I would say for sure, you probably even gained a little more than that. If you can see the difference, you can be sure that the changes are beginning to occur. It's even possible that you lost a few extra pounds of body fat and gained closer to 4 lbs muscle in a four month period.

I would like to add though that I wouldn't put too much faith in body fat scales either, the problem with them, is that most of them simply send pulses through your feet so the current will only account of the amount of body fat in the lower half of your body and it will estimate the rest of your upper body fat % based on some of the information you plug in at the beginning. So if you carry more fat on the lower half of your body then your % will read a higher estimation. As far as I understand it.

The best kinds are the ones where you step on the scale and you hold onto hand held devices at the same time. They're a bit more accurate.

I've physically watched myself get leaner, but the scale I have doesn't seem to reflect the changes I'm seeing. Hydration levels will screw with it too. I guess my point it, I feel like my progress isn't really showing up on the scale, but it is showing up in the mirror. 

Ank: You have to do a little math.  You started at 110 with a 19% body fat which works out to 20.9 lbs of fat on 89.1 of bone and muscle (and other stuff).  You have moved to 112 with 18% body fat which works out to 20.2 lbs of body fat on 91.8 lbs of all the other stuff.  That presumably means that you have a) shed almost 3/4 pound of fat and b) gained over 2 pounds of muscle.  These are only estimates of course but they show pretty significant improvements.