I would go ahead and get the amnio. Usually women choose not too because the pregnancy is not far enough along at 20 weeks for the baby to survive if the needle created a leak in the amniotic fluid. You are asked to get an amnio to make sure the lungs are developed.
During my last pregnancy, I wanted a c-section at 37 weeks because I had the HELLP syndrome (serious form of preeclamsia) with my second pregnancy and I did not want a repeat performance of that. My doctor requesting an amnio before the surgery and found that the baby's lungs were inconclusive, meaning that they could be developed enough or not. He made me wait another week to make sure the lungs were developed. She is a happy, healthy 17 month old today.
The procedure was, if anything, interesting and not that painful.
Good luck to you!