You sound like your head is screwed on right and you know what you want and most importantly you're not being taken in hook line and sinker by what she is saying - you're right to doubt and to ask questions about this. Shoulder dystocia is by and large a positional problem - 5lb babies can cause problems if they are not in the *best* position for birth. It's a great start that you have a doula and she is offering you support and literature about this - she will be there to help coach you through the birth and if babe does turn out to be big - she will be more than able to guide you into the best positions for it.

I have had two babies (out of 4) that presented with shoulder dystocia - one was 4.32kg and my last was 4.56kg - but they were BOTH in a terrible position for birth and they have very broad shoulders, so both those factors combined to make it harder to get them out, but not impossible - I didn't even tear with my last baby, just the smallest graze. Trust your instincts on this one.