Congrats on your pregnancy!
Last time I was pregnant, I paid more attention to nutrients then calories/points. I had just lost a bunch of weight from my first son when I became pregnant with my second. I was a healthy weight (BMI 22.8) and managed to keep it at 25 pounds gain even.
What I did worked really well for me. I drank TONS of water every day. I weighed myself every day to make sure I was keeping track. If I gained more than a couple of pounds in a week, I'd pay better attention to my eating habits and make sure I wasn't eating a lot of sugar or empty calories. If I went a couple weeks without gaining weight (once I was in the 2nd tri), I would make an effort to eat more and to eat more calorie-dense food. I lost weight in the beginning bc of morning sickness (toast, toast and more toast, anyone?) and then leveled back up & gained my 25 from there. I tried to eat TONS of protein every day, and made sure I chose whole grains and ate whatever fruits and veggies I could. I was vegetarian (except a few episodes of momentary insanity) and ate a lot of dairy. I couldn't eat any sugar (even fruit) for a long time, because it made me really queasy really quickly. Fortunately sugar isn't full of nutrients I HAD to have.
Most of the time, besides keeping an eye on my weight (I was trying to gain at LEAST 25 for a healthy baby and was trying not to gain more than 35) as I mentioned above, I mostly kept track of water intake, and protein. A lot of protein. Read the Brewer's diet site & tried to loosely follow the suggestions there for types and amounts of protein.
Hope this helps!
ETA: Oh, and I stayed really active to make sure I was giving myself the best health for my VBAC and moving a lot to keep baby in good postition with gravity per Spinningbabies.
From everything I've heard, 20-25 pounds is the bare min. That is the approx. weight of the increased maternal blood volume, some needed fat stores, 'water' weight, the weight of the baby, placenta, the anmiotic fluid.