The extra soft, naturally shaped nipple makes it easier to switch between breast and bottle feeding, and promotes healthy, active feeding. The built-in valve flexes with baby's natural feeding rhythm and lets baby control the flow of milk, which can help reduce the risk of overeating and spit-up. The valve also prevents a vacuum from forming in the bottle to keep air out of baby's tummy. This reduces colic and gas after feeding. Slow Flow Rate is ideal for babies who are one month or older or breastfed babies of all ages.