Posted by a reviewer on Amazon

Every 6 bottles or so one leaks

Posted by a reviewer on Amazon

You can actually see the bubbles disappearing in the bottle

Posted by a reviewer on Amazon

You can also hear the air coming out

Posted by a reviewer on Amazon

The nipples on our philips avent natural are just too stiff and dont fit tightly in the collars so they leak

Posted by a reviewer on Amazon

Plus with smaller nipples or especially when the bottles are capped the bottle is really a vacuum so milk shouldnt leak through the bottom unless its significantly enlarged or you received a defective bottle

Posted by a reviewer on Amazon

They seem too tight and dont vent very well causing air to enter through the nipple tip

Posted by a reviewer on Amazon

Also the threading on the collar is odd and if its not aligned just right or if its a little too tight itll cause leakage

Posted by a reviewer on Amazon

Milk leaks through the air vent making the nipple the only hole into the bottle so you get air bubbles that this design was meant to prevent