I HATED these bottles. It didn't help we didn't read the instructions properly before using, but who would have thought you needed to read instructions before using a baby bottle! The vent at the bottom was not letting air flow properly, my son literally fussed and cried for 20 mins before we realized he wasn't getting any milk; and then after re-reading the instructions, they still didn't work. I ended up returning them for Avent bottles which are FANTASTIC!