The Spectra Disposable Milk Collection Bags are double-lined providing a sturdy and spill-proof structure. Each Spectra Disposable Milk Collection Bag when filled with breastmilk, will stand upright allowing accurate readings from the measurement scale. After filling a milk bag, Spectra highly recommends labeling each bag with the correct date of use which is located on the front. It’s important to remember to leave at least 1 inch of milk from the seal when storing bags in the freezer as breastmilk will expand as it freezes.