It all depends on the person and the pregnancy, Britt.
I categorically refuse to worry about my weight while I am pregnant. I use it as a time to practice eating to hunger, & IMO that's an incredibly important skill for real life.

I tend to follow a pattern of losing the first trimester, maintaining the second (or part of it at least), & gaining in the third.
You have to keep in mind too that a lot of the weight gain during pregnancy is a phantom. I weighed 242 the day I had my first daughter, but when I went in a week later to get my staples pulled, I weighed 219! (And no, I have no idea why they weighed me before destapling me.

) Trust me, amniotic fluid gives a whole new spin to the term "water weight gain".
ETA: Not all diet programs are safe to continue while pregnant, especially if you've grown accustomed to eating only a little bit. Atkins used to say that NO phase of his program was advisable for pregnant or nursing women, but his company has relaxed that to say Lifetime Maintenance is OK. Definitely something to review with your doctor or midwife.