Anatomy & physiology | Hospital Infant Feeding Network

ANATOMY & PHYSIOLOGY - KEY FACTS

Colostrum is the first milk produced (lactogenesis I), before milk 'comes in' (lactogenesis II) on average 2.5 days after birth (range 1-4 days). Colostrum is concentrated and contains more immunoprotective compounds than mature milk (for example IgA, lysozyme and lactoferrin)

Image Credit: NCI NIH 2010

Nipple stimulation and milk drainage stimulate both milk release and hormone receptor site activation. Central (prolactin release) and local mechanisms are involved

When the mammary tissue becomes full with milk, inhibitory hormones are activated to reduce further milk being synthesised and released

There is evidence for a critical window to establish a robust milk supply in the first few weeks of life - therefore if effective breastfeeding/chestfeeding is compromised in this time period it may have a long term impact

Newborns feed relatively small volumes in the first few days of life - a healthy baby delivered vaginally feeds on average approximately 6ml/kg/day on day one and 25ml/kg/day on day two (although the range of normal intake is very wide). A systematic review of studies using test weighing suggests an average intake of 21.5ml in 24 hours on day 1 of life and 100.3ml in 24 hours on day 2 of life for directly breastfed healthy babies (note this is not per kg but total volume).

Milk is synthesised continuously and is stored in the breast/chest until the baby next feeds. An analogy for parents is that it is more like a river than a lake. During most feeds babies take in only two thirds of the immediately available milk

The amount of milk that can be stored in the breast/chest before inhibitory feedback loops are activated varies between people, and over time for each person. Storage capacity in both breasts/sets of chest tissue can range from 75 ml to 400ml

Volume of milk produced rises steeply in the first 1-2 weeks of life and then remains relatively stable throughout the period of exclusive breastfeeding/chestfeeding. The average amount of milk produced when exclusively breastfeeding/chestfeeding one baby is 700-800ml per day (although for some normally growing babies, daily volume may be as low as 450ml). Systematic review of weight based reports estimates that milk intake increases rapidly to a maximum of 150ml/kg/day by week three of life and then declines gradually to around 100ml/kg/day by six months of age.

Frequency of feeds during the period of exclusive breastfeeding/chestfeeding can range from 4 to 13 sessions per day (a session arbitrarily defined in that study as one or more feeds where the gap between feeds is less than 30 mins). The average was 8 sessions per day, in this study and systematic review. Volume taken in is also highly variable, both between babies and at different times of day for the same baby

The milk taken in at the start of a feed has lower fat content than at the end of a feed (foremilk and hindmilk). However a baby tending to feed frequently will have a less pronounced fat gradient than a baby tending to feed infrequently - so total fat intake is the same. Telling someone to try and drain the breast to get to the fatty hindmilk is therefore not required, and could reduce supply overall if milk is building up on the other side and activating inhibitory feedback loops. Putting the baby back on the same side repeatedly ("block feeding") is in fact a method for dealing with oversupply and is likely to be counter-productive in babies with faltering growth

After complementary feeds are introduced, milk volume starts to reduce. Older breastfeeding/chestfeeding babies and children were drinking around 100-400ml in 24 hours in some studies. Toddlers often feed frequently but this does not necessarily mean they are taking in large volumes of milk. The composition of human milk changes over time - generally after a year it has higher protein and energy content.