I have used Dr Browns with both of my children. I love everything about them except this little piece. For some reason I dont ever remember it being a problem with my first child but with our second they have started collecting mold in the middle of them. We purchased all new ones for her so I dont know what is going on with it. With my sons the vent inserts were different and had little indentions inside them but with my daughters they are more smooth in the middle which I considered to be a good thing because it didnt seem like milk could collect in there as easy. I have tried bleaching and boiling and washing but nothing will remove this debris. I just think its really gross and dont want my daughter getting sick because of this so we have ended up throwing most of them away and using it without them. We love all of the other parts of this bottle and the concept is smart but something needs to be done about this.