wash it, throw it in the dryer and it will be fine. I have never used poly batting (I can't handle the feel when I have rough hands) and I never prewash anything! It does give it that wrinkly used look!
All of these were done with W&N the first two were not washed yet when the pictures were taken but the others were. I washed them and then dried on high to get max. shrinkage/wrinkling!
http://www.quiltingboard.com/t-24590-1.htm
Good luck and Congratulations on the baby!