I wouldn't do a diet, but I'd eat differently. It's what I do. The more protein you eat, the less you gain. The more carbs you eat, the more you will gain. Fruits/veggies don't count as carbs...so they are "free" food. Salads are free foods. You can eat whatever you want of those things. They are very healthy for you too!
Limit your breads (even whole wheat), juice (anything that's not water), rice, potatoes, chips, any unhealthy snacks, etc...all those things will make you gain weight.
Eat more protein. Beef is especially important in pregnancy (contrary to popular belief, it's not bad for you if you eat pure, healthy, organic beef. There is so much stuff in there that is important for you). I go for organic beef. But it helps with your blood as well as give you strength and energy for the pregnancy. Eat chicken and whatever other meats you like. There are other types of protein too, though. It doesn't have to all be meat. But you want 80-100 grams of protein a day.
With my first two pregnancies I didn't do this. I didn't have any sort of concept of what carbs did. I ate them as I pleased. I thought I was eating healthy versions of them, but the truth is, carbs make you gain weight - even if they are whole wheat carbs! Carbs mess with your insulin (even in non-diabetics).
I'm 31 weeks pregnant and probably 8 weeks ago I really started eating like this more strictly. I was gaining weight too fast - for no reason. So, I started eating more protein, few carbs - and lots of veggies/fruits. For snacks I eat an apple or grapes or banana. Those help a lot to give energy. They also have a lot of nutrients in them that are important. Some people say not to eat fruit if you don't want to gain weight, but I disagree! Fruits are so important. Oh, but over the past 8 weeks or so, I've maybe gained 2 lbs. As I said, I was getting a little chubby (for me), so it's been good that I've backed off on the weight gain.
This "diet" is good for any time. You will lose weight when you back way off on carbs. Make sure to drink only water. Juice isn't healthy for you, even if it's 100% juice (the way our bodies digest juice is more like you're drinking sugar...but if it's in fruit form, you chew it and your saliva breaks it down. In juice form, that doesn't happen!).
Anyway, so that's what I'd suggest:-) I don't count calories. I eat when I'm hungry. I just pay attention to the protein/carb thing. I make all my meals from scratch. Oh, and I also clean a lot (the nesting stage has gotten to me!). The more active you are, the more it'll help you not gain weight and will help your metabolism.
Good luck!
ADDED: The reason for the 11 lb weight gain COULD be because of water retention or something. Sometimes things like that happen and it doesn't mean you're being a slob or anything! I think often people don't know what really makes them gain weight...so it's hard to control it.