Size: Pump In Style The Pump In Style Advanced Breast pump Starter Set is part of Medela's patented 2-Phase Expression technology pump family. Why are Medela pumps different? Because Medela breastpumps have the original 2-Phase Expression® technology which, using the one-touch let down button, has been shown to produce more milk in less time.* Medela is the only company to provide proven, research-based pump technology that mimics a baby's natural nursing pattern. Through research, Medela learned that there are two distinct phases of how babies breastfeed. Stimulation phase - when babies first go to breast, they suck faster to start milk flowing. Expression phase - after milk flow or "let down" starts, babies breastfeed with a slower deep suck to remove milk. The original 2-Phase Expression technology can only be found in Medela breastpumps. Medela offers a wide range of breastshield sizes. Using the right breastshield size is important to: Ensure proper suction Experience optimal milk removal. Achieve maximum pumping comfort. *When pumping at Maximum Comfort Vacuum in the Expression phase. NOTE: Box says "Not for retail sale, for insurance purposes only"