Because most medications cannot be tested on pregnant women for obvious reasons. Babies are born addicted to nicotine, marj., meth and prescription drugs. He is telling you no because it is considered harmful. I have never heard it is ok as long as you stop at the end to minimize withdrawls. They are already addicted by then. I would let your dr. know exactly how you are feeling and the pain you are experiencing to see if he has other suggestions. The only medication approved during pregnancy for pain is tylenol. In all honesty, you are either going to take the Ultram and risk addiction/side effects to baby or rest as much as possible to stay as comfortable as possible during this pregnancy. The bigger the baby gets the more you will need to rest and take the weight off your back. I wish you the best.
You NEED to speak with a professional about it and if they advise against it then ask them for alternatives.
A category C medicine isn't nessesarly dangerous. I was on one for 6 months during my pregnancy and taking it 2-4 times a day. I would definitely get a second opinion from another doctor, or you can call your local pharmacy. They have more knowledge of drugs than doctors do and can give you proper advice.