This connector has been completely critical to my exclusive pumping success with my now 6 month old. My flange size is a 13 and I use this adapter to connect to my Spectra S2 pump with a size 13 half shield attachment (use Maymom for this). I previously used 15s before this adapter was an option and could only pump about 60% of the volume. Now, I pump without any pain and at a higher output because I can pump with my real flange size. SO thankful for this adapter!! Alexandra was also extremely helpful and detailed in helping me figure out what I need for my pumping set-up when I sent an e-mail to customer service. Connects to my 9 ounce AVENT bottles directly, which is also convenient. Also works with other spectra compatible bottles.