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5.2 Breastmilk transfer

Obviously the goal of breastfeeding is to achieve milk transfer from mother to baby.

Breastmilk transfer occurs by:

  1. positive pressure as the milk ejection reflex (MER) forces milk from storage in the alveoli towards the nipple, and
  2. negative pressure, created in the baby's mouth, drawing the milk from the breast into the oro-pharynx.1

Understanding the anatomy


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Imagine the functional tissue of the breast as like a large bunch of grapes. The grapes are linked by thin stalks to thicker stalks and thicker stalks again as more sections of grapes join it, until there is one thick stalk.

A grape represents an alveolus (pl. alveoli) which holds the breastmilk. The stalks are the ductules and lactiferous ducts through which the milk passes, and the whole bunch of grapes represents one lobe of functional breast tissue. Each breast contains about nine lobes.2


The lactocytes, columnar epithelial cells that make up the alveolus, synthesise the breastmilk which is then stored in the lumen.

Each alveolus is surrounded by a basket-like weave of myoepithelial cells, muscle cells. Contraction of the myoepithelial cells constricts the alveoli, forcing the breastmilk into the ducts and towards the nipple where the baby can remove it.


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Workbook Activity 5.4

Complete Activity 5.4 in your workbook.

Milk ejection reflex

In response to stimulation around the nipple/areola area a surge of oxytocin is released from the posterior pituitary gland. It travels via the blood stream to both breasts causing the myoepithelial cells surrounding the alveoli to contract. Sometimes this can be so forceful as to cause the milk to drip or even squirt from the nipple.

This is called the milk ejection reflex (MER).


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MER can be quite forceful.

MER can be quite forceful.
© A.VerSteeg


When the MER has been stimulated mothers may describe the sensation as

  • a sharp, momentary pain in the breast, or
  • a fullness or tightness of the breasts, or
  • a tingling sensation inside the breast, or
  • there may be no sensation at all.

It can be seen to be functioning when milk drips or squirts from the breast. If the baby is suckling at the time you'll notice a change in suck pattern to a suck / swallow / breathe / suck / swallow / breathe pattern with swallows about every second. This vigorous sucking from increased milk flow may last 1.5 - 2 minutes. You may hear the baby gulping the larger volume of milk too.

As the effect of oxytocin wears off the alveoli and ductal system, milk which is not removed by the infant (or by expressing) moves back up the ductal system to the alveoli once more.

Take home message:

Milk transfer is dependent on milk ejection! No MER = Hungry baby

Important

Many mothers do not feel the milk ejection at all, and rarely is it felt in the first few days postpartum despite it functioning quite well. Prior to the milk 'coming in' the change in the baby's suckling is also not noticeable.

In multiparous women involution pain is felt more acutely during milk ejection. (Why?)

Negative pressure transfer

The mouth of the newborn is ideally suited to suckling at the breast. The tongue grasps the breast tissue and the lips create a seal around the areola. Negative pressure in baby's mouth draws the breast into position and maintains it there forming a teat of soft breast which completely fills the mouth. The small mandible fits closely against the breast, allowing the baby's nose to be free for breathing, while the buccal fat pads in their cheeks prevent the cheek from 'collapsing' in during suckling.

Negative pressure is created in the sealed mouth when the back of the tongue drops in response to the increased milk flow at milk ejection. There is a coincident lowering of the soft palate. The intra oral vacuum is at its greatest when the tongue is fully lowered prior to swallowing. It is this negative pressure which draws the breastmilk from the nipple into the baby's mouth.

Workbook Activity 5.5

Complete Activity 5.5 in your workbook.

What should I remember?

  • The two mechanisms which are required for milk transfer during breastfeeding - describe each.
  • The hormone which stimulates the milk ejection reflex.
  • The mechanism and function of the milk ejection reflex.
  • The changes which occur in the mother and the infant during MER.

Self-test Quiz

Notes

  1. # Ramsay D et al. (2004-09-10) Ultrasound imaging of the sucking mechanics of the breastfeeding infant
  2. # Geddes DT (2007) Inside the lactating breast: The latest anatomy research