I agree, prewash the flannel, it will shrink. As for the fleece, it's extremely warm! It is polyester, so it won't shrink. But it also stretches more than cotton or flannel, so just be careful not to pull on it when you are stitching. Also, with the fleece, it can help to pre-wash it, so that any stretching it did on the bolt has a chance to be corrected before you use it.