A must for back up!10/13/2015 ByFirst Time Mommy I don't know about you, but my pump came with two measly bottles. I've ended up buying this four pack twice as having the extra bottles has proved essential. I can have a couple full in the fridge and still pack the six bottles I need for pumping at work. (Three times a day, both breasts at once is my pumping routine.) It makes rotating and cleaning much easier since I always have some clean bottles on hand. I was reluctant to get spare parts as I'm all about keeping it simple and not getting more than you need, but these are a must. Mommy gear was one of the few places I could find that carries this brand. They have been a breeze to deal with - super fast delivery and great products.