PRO PARENT TIP: Please note that we only sell size S slow flow and SS Super Slow Flow. S is the size that was included with your Spectra Breast Pump. Since breastfed babies have superior jaw development, this should be the only size you need. Nipples also increase in flow as they age, so you may want to replace your baby's nipples as often as every 6-12 weeks, depending on the number you have to rotate. You will find the flow marked as S on the bottom of your nipples. The number shown on the bottom of the nipple is not the size, but instead a lot number for production reference only. Super slow flow is marked SS.