Is that a leak?
Because the Dr. Brown's bottle is fully-vented it allows positive-pressure flow, similar to breastfeeding. This is essential for the baby to feed without having to suck against a vacuum. Under certain conditions the breastmilk or formula can exit through the vent. For example if the liquid is overheated the expanding air inside the bottle pushes the liquid through the vent causing leakage. Or if the bottle is shaken to mix formula, which we do not recommend, the liquid may enter the vent.
When liquid gets into the venting system, it can sometimes leak out around the nipple collar. This can happen three ways:
If you heat your bottle with the nipple collar securely in place, the liquid will be forced up into the vent as it expands. To avoid this, either heat the bottle without the nipple collar or loosen the nipple collar one-half turn after heating to allow the pressure to equalize. Then, re-tighten it before feeding.
If you use powdered formula and mix it by shaking the bottle, liquid can get into the vent. The solution for this is to mix the formula by stirring it with a spoon rather than by shaking.
If you overheat the liquid in the bottle, the vapor will rise and condense in the vent. To prevent this, avoid warming the liquid beyond body temperature.