Hi there, Annie17 and Jeannette51! Good to e-meet you. Fusible battings have a special type of water soluble fusing, unlike fusible interfacing fabric used to interline clothing collars and cuffs, for example. While clothing fusibles are meant to stay secure during repeated washings and are water impervious, the fusing adhesives used on fusible quilt battings are intentionally water soluble, so that they may be washed out of your quilt, once the quilt has been quilted. When fusible battings are left in place, the finished quilt has more body, which some quilters prefer for display. But, for softness and ease of draping, the fusible adhesive is gently washed out in cold water, if the quilt is going to be used on a bed or comfortably wrapped around a loved one.
So, to answer your questions, if you prewash a fusible quilt batt, the fusible adhesive will be washed out. The manufacturers have anticipated your concern over shrinkage, however, so cotton fusible batts are usually prewashed. Always good to check with the manufacturer, though, just to make sure. Fusible polyester batts don't shrink, so prewashing would not be necessary.
As a Fine Arts Conservator who works with textiles in museums, I would be remiss not to mention that if you are investing your time and money into making a fine art heirloom quilt to last for many generations, not using any adhesive chemicals on your fine cotton fabrics and batting would be best. It is a rule of thumb that the fewer chemicals introduced into a textile, the longer it will last in appearance and structure. Pinning and basting are the best methods for museum quality quilted art or bed coverings. I am always happy to answer questions, if you think of something else. I can be reached at my Quilting Board Profile
http://bit.ly/11hmYBV , on Twitter as @SunbonSmart or on Facebook
http://on.fb.me/Z9rBMb , whatever's easiest for you. I love working with quilters! Much Love, Fondly, Robin