Given that medela don't mention these things in their calma bottle lid packages (I found out after contacting them about it) I thought I would warn parents that use them. They only have a use life of 1-3 months apparently. They also only work with breast milk. If you use them with formula(we didn't) or past a couple of months (we did) the membrane breaks. Attached is some pictures to show the problem. Because we didn't know of this issue one of them broke completely and we can only assume our son swallowed it. Upon seeing the damage I checked our other lids and found 2 others cracking. So please if you use them regularly check the membranes for cracks and then get rid of them before they fully break. We got lucky that the membrane breaking off caused our son no issue.