This is a recent post by highly regarded Dr. Jack Newman on an VERY important topic you should know about (before the birth of your baby).
"This is post about test weights (weighing a baby before and after a feeding to see how much he got). I disagree with this approach which, at first glance, seems scientific and accurate. But it’s not. In the first place, see my previous post about percent weight loss and scales. Okay, test weights are done on the same scale, often scales made for testing weights, so they are likely to be accurate. But there is more to be concerned about.
Consider this: A baby who is exclusively breastfeeding and gaining weight well at 5 months is not getting any more milk than a 1 month old who is exclusively breastfed and gaining weight well even though the 5 month old weighs at least twice as much as the one month old. So how much is a baby supposed to get from the breast even if the weights are accurate? We don’t know. Breastmilk is magic and defies the logic of the “bean counters”.
In any case, the amount that is calculated that the baby “needs” is based on what a formula feeding baby would “need”. But even calculating on the basis of what formula fed babies need does not make sense because even formula fed babies do not always drink the same amount.
On top of that most mothers will agree that they have more milk in the morning than in the evening. So if the milk intake is measured in the morning, the result may be falsely reassuring. If measured in the late afternoon or evening, the result may be falsely concerning.
It is well known that the amount of fat in breastmilk is variable so that 30 ml (one ounce) of high fat milk may be a lot more satisfying than 30 ml lower fat milk and just as adequate for the baby’s growth as considerably more formula. And I don't me "hind milk". Studies also show that breastmilk fat content varies throughout the day.
It is also well known that anxiety can decrease the milk ejection reflex and thus, how much milk the baby will get. Being “put to the test” of test weighings can be very anxiety producing and affect the intake of milk by the baby.
So the best way to know if a baby is drinking or not drink from the breast is to watch the baby on the breast and watch for the pause in the chin. The links for the videos follow in the comments."