I have be use these for about a week now . I be exclusively pump at the moment for my Preemie and the NICU recommend I use storage bag instead of invest in storage bottle . I have not have a issue like some say with leak . then again I put mine directly in the freezer after pump . I have never see mine leak once yet . I can not say how they be if keep in a diaper bag or anything after thaw but then again how much of anything liquid would fare well in a ziplock type baggy if be carry around ?? I will keep buy these becuase they work for I .