First, don't be terrified to gain weight while you are pregnant. The baby needs you to gain that weight! You can gain too much weight, but if you are conscious of your daily intake it really shouldn't be a problem.
I followed SB principals during my last pregnancy (my newest is 3 weeks) and gained 12 lbs. (I was overweight to begin with and 12 lbs. was right on par with what the doctors wanted). I added more grains and dairy during the pregnancy, but basically followed Phase 2.
Now that I'm concerned about my waistline again while still concerned about my milk production I've been following a modified Phase 2 (my menu for today is posted in the daily chat for today) - I add more dairy (and use full fat yogurt instead of fat free, also 2% milk instead of skim or 1%), more grains (I try to have 2-3 servings per day), and just more food overall (I'm striving to hit 2300 calories per day) but the foods are all in line with SB principals.
I've lost the 12 lbs. that I gained with the pregnancy and an additional 4 so far. My main focus right now is not on weight loss but on building my milk supply.
But again, with the right selection of foods and exercise during your pregnancy (and you can exercise your entire pregnancy unless there is some complication that would prevent it - I jogged during my pregnancy) you can keep the excess weight gain to a minimum and taking it off afterwards will be so much easier!
Also, my DS1 eats basically the same foods that I do except we just add more whole grains, fruits and full fat dairy instead of reduced fat. He gets treats, but rarely. He will often turn down ice cream or cookies - I have to wonder what is wrong with him!
Thankfully for him, he has DH's genes and will forever be slender.