((Krazibean))
Pregnancy leave is an individual decision. How much time your T will take off can only be answered by her. I think you should go ahead and tell her your worries. They are valid concerns and I don't blame you a bit for being jealous. But I bet T will still care for and love you just the same as she does now.
As for her pregnancy affecting her ability to concentrate, I don't think that will be the case. I worked through all three of my pregnancies and didn't notice any difference in this area.
Don't worry, but do speak with T about this, so you can prepare for any time off she may take.
Peace