Baby bottle nipples come in three to four different levels, or speeds, to accommodate how fast your baby feeds. Newborns start with the lowest level and move up every three to four months until they are at the fastest level. After they have reached the fastest level, then they will move up to a sippy cup around 1 year old or slightly after.
The lowest level for nipples is the "slow" speed. When buying these nipples, the package should say "slow flow." The nipple itself may also have the word "slow" on the bottom of it, at the base ring. This way if the nipples are mixed up, you'll still know what level it is. When your baby is first born, he cannot eat very fast and only needs about 2 oz. or less per feeding. This is why the slow nipple is used; otherwise your baby can be overwhelmed and spit out all the milk. Your baby will use this size nipple for approximately three months.
The next level for nipples is medium. These should also have the word "medium flow" on the packaging and the word "medium" on the bottom base ring. Your baby will use this nipple from 3 months to approximately 6 or 7 months old. Your baby eats anywhere from 4 to 6 oz. during this time at each feeding. All babies are different, though, and as they hit their growth spurts, some may want to eat more.
Fast-level nipples are the final category. On the packaging, these should say "fast flow" and "fast" on the bottom base ring. This nipple is used when your baby is 6 to 7 months old or older. Your baby will be eating 7 oz. or more per feeding by this point. This is also around the time you'll start introducing baby foods and cereals. The cereals can be mixed with the breast milk or formula to be fed through the bottle, though this is often no longer recommended.
Level 1 to 4
Some brands of nipples use numbers instead of words. One brand that uses a numbering system is Avent. On these nipples, the number is at the bottom of the nipple, but it is still visible on the outside of the bottle, whereas the label for the other brands is only visible on the inside, since it is on the bottom base ring. The levels are just like slow, medium and fast, except that there are four levels instead of three. Level No. 1 is the slowest and No. 4 is the fastest for this brand. If you are using these nipples, you would move up in levels a little faster, probably every two to three months.