DESC: The MAM UltiVent nipple lets you customize the flow of your UlitVent bottle system for your growing baby. Designed to draw milk from the nipple just like during breast-feeding, this ultra-soft silicone double pack helps transition between breast feeding and bottle feeding. UltiVent Nipple Sizes: 1 = slow flow: newborn and ideal for breast milk; 2 = medium flow: for water & breastmilk, formula; 3 = fast flow: formula, breastmilk; X = cross cut: thicker liquids. Two pack.