I am in my second trimester and also am using Monistat 7. My midwife prescribed it for me. The seven day is a much better choice than the 3 or 1 day versions. And also much better then the oral pill treatments they have. I was worried too, but found that the seven day is considered pretty mild and not a problem for pregnancy.
There are some things you can also do with diet to cut down on yeast. Goggle it and the web has lots of info about foods to avoid. Cut all the processed sugar foods and really yeasty breads. However, I personally would not follow the diet completely while pregnant, the babies balanced nutrition comes first. For example I still eat lots of fruit, even though it has lots of sugar, which feeds the yeast. You should also eat lots of high quality yogurt and perhaps take extra acidophilus pills. That will help keep the yeast down.
From what I've heard its worst to have extra yeast during birth because you can pass thrush between you and the baby.
Good Luck,
T.