I went into labor at 33 weeks with my daughter and they did an amnio to test for her lung development. As the baby's lungs mature, the surfactant can be measured in the amniotic fluid. I was scared about the amnio, but I honestly can't remember a thing about it - I don't even remember that it hurt. Also, there had been research that suggested that having an amnio can put a mom into pre-term labor, but they have since found this to be un-true.
In your case, I don't see why the docs would want you to deliver early. There is nothing wrong with having a big baby, especially if you are already planning a c-section. Has your doc explained she would want to deliver early if your baby is big? There is a reason why we carry our babies for 40 weeks or so - that's how long it takes for them to finish developing before they are ready to be outside the comfort of your body. As a mom of a preemie, I would VERY HIGHLY RECOMMEND that you obtain a concrete medical reason for delivering your baby early, and follow up with a second opinion. Once you are well informed, you make the decision based on what feels right to you.
Also, I am currently pregnant with our second child and had a checkup two days ago. In the ultrasound, our baby was measuring at a nice 70% for his gestational age at 24 weeks. When the nurse measured my fundal height a couple minutes later, I was measuring at 26 weeks. The size of my belly was not an accurate reflection of the size of the baby. If the doc thinks your baby is big, you may want to request an ultrasound to validate the suspicion of a big baby.
Good luck to you - I'll keep my fingers crossed for a nice healthy 40 weeker. :)