If I start now, I might be done by October!

So, my plan: I want to knit myself a sweater. However, sweater weather here is pretty much over, and by the time it rolls around again, I expect to be seven or eight months pregnant (if all goes well, and depending on when we have our first chilly nights, whether I visit anyone up in the mountains, etc.). So I looked around online for maternity-sweater patterns, and all of them only go up to about size 20. That's fine for the bottoms, but on top I am more like a 26. How do I modify from 20 to 26? I am probably going to work off of this pattern. It has numbers in parentheses for each increasing size, but I don't know where and how many stitches to add, where to stay the same, etc. I have a pretty long torso, if that makes any difference.