I tested a lot of other brands have quickly finished by deform and yellow ... this milk leakage causing very unpleasant and sometimes embarrassing. Then I adopted them and they are fine. Good stimulation of milk comes with openings to allow the skin to breathe and wick moisture. the silicon does not move and does not yellow. I used these shells, they were used for my two children and they have not moved. (The same for the pair 2) I also use the manual breast pump Advent and really they are very good quality products that make life easier for moms.