If your baby is latched onto your breast, that means he's eating, right? Well, sometimes.
As a new mom, it's not unusual for you to assume that once your baby has latched on properly, he's happily filling himself up. You'll feed him for what seems like an hour, but then 20 minutes later he'll be hungry again.
What's happening is that your baby is just "being at" your breast rather than feeding from it. Sure, he's sucking, but that motion is merely comforting and mechanical; he isn't actually swallowing any milk. Many times, he'll nurse himself right to sleep without filling up at all.
To make sure your baby is eating when he's nursing, it's important to learn the difference between a suck and a swallow. This may be tough in the first few days because your baby swallows colostrum—the milk you produce after giving birth—less frequently than mature milk. However, you should keep watching for signs of swallowing. After your milk comes in, his swallowing might become more obvious as your milk supply gets bigger and flows faster, making it easier for him to eat.
After you learn to recognize what your baby's swallowing looks like, it's a good idea to watch his swallowing pattern. Effective feeding is defined as swallowing after 1, 2 or 3 sucks and doing so in long bursts, such as 10 or more.
Source: Heather Kelly is an International Board Certified Lactation Consultant (IBCLC)