Lactiferous duct diameters

At rest

• Milk duct diameters are highly variable, in the one woman and between women. Ducts have been imaged by ultrasound between 0.1 mm and 10 mm diameters at rest - 100 fold difference in diameter. (Ultrasound imaging can resolve down to 0.1mm; finer ducts aren't visualisable by ultrasound.)

During ductal dilation

• Ductal diameter increase is highly variable by a factor of nearly fourfold between ducts, ranging from 0.5 to 1.9 mm. Average increase in duct diameter with milk ejection is half a millimetre.

• Ducts still dilate when either very small or zero volumes of milk are being release.

• A backwards flow after peak duct diameter is observed in the non-expressed breast.

• Peak duct diameter is higher for the first milk ejection compared to subsequent. Increase in duct diameter varies from 76% with first breast to 40% increase with second breast in the one feed.

• The initial milk ejection has the highest increase in intraductal pressure, with subsequent milk ejections producing lower pressures.

The relationship between ductal dilations, milk volumes, and flow rates

• The first milk ejection contributes a greater volume than subsequent milk ejections. One study found that the first two milk ejections produce 62% of total expression volume or milk intake. Another study found that the first two milk ejections resulted in 76% of total milk transfer. Between 27 to 45% of the total amount of milk extracted occurred during the initial letdown.

• As the amount of available milk in the breast decreases over time, the milk ejection episodes yield less milk. But duct diameter changes are the same as at the start.

• Peak duct diameter is independent of the amount of milk within the breast.

• In 39% of women with multiple milk ejections, the baby ended the feed in the middle of a letdown, showing baby's capacity for appetite regulation.

• Rate of milk flow during ejections relates to both strength of pump vacuum and fullness of breast. Rate of flow during expression period is extremely variable ranging from 0-4.6gm/5 seconds.

• The flow rate of milk varies between tremendously, between 0.6-4.6 mls/5 seconds.

Frequency of milk ejections

• Although 88% of women feel their first letdown, 12% don't, and most don't feel any other letdowns in a feed.

• A range of 0-9 letdowns has been observed during breastfeeding for 15 minute period. One study showed that the number of letdowns one woman has within a period of time may be double that of another woman e.g. 3.1 to 7.1 letdowns in 15 minutes. Another showed a range of 1-12 letdowns in a 15 minute period, with a mean of 4.3 letdowns. A range of 1-17 letdowns (ductal dilations) has been observed in breastfeeds of up to 25 minutes duration.

• The number of milk ejections a woman has during an episode of milk removal remains stable over the course of lactation. For this reason, women have their own unique time that the baby or pump takes to remove milk, and this is stable over time.

• Each woman's timing, pattern and number of milk ejections (her milk ejection profile)) remains consistent during different lactations.

Latent periods prior to or between milk ejections

• The time to the first milk ejection is 56 seconds on average, with a range between 23-79 seconds in normally breastfeeding women. Some normally breastfeeding women take 120 seconds before first letdown. That is, the initial latent period is more than three times longer in some women than others.

The suck rate prior to initial letdown varies between 72 - 120 sucks/minute

• Prior to the initial milk ejection, a baby may transfer up to 30 mls of milk, which is resting in the ducts of that breast.

• Time since last breastfeed does not influence time taken to the first milk ejection.

• There is a minimum latency period of 40 seconds between milk ejections, with a latency range of 40-203 seconds. The median time from the end of one milk ejection to the beginning of the next is 90 seconds.

Durations of milk ejections

• Milk ejection lengths range from 100-800 seconds. Average duration of milk ejection was 228 seconds, but highly variable.

• Strength of baby's intraoral vacuum has no effect on duration of the milk ejections.

Milk ejections and oxytocin

• Varying levels of oxytocin result in the milk ejection reflex. Oxytocin levels remain stable up to 12 months of lactation, even though milk production decreases as solids are introduced.

• Each woman releases the same amount of oxytocin each milk ejection, but the amount varies between mothers.

• Breastfeeding induces an immediate and short-lasting oxytocin release, typically lasting 20 minutes. Early postpartum, oxytocin release is pulsatile, occurring 5 pulses/10 minutes, and but this becomes more a protracted elevation as lactation progresses.

• Alveoli contract asynchronously and milk ducts dilate asynchronously in response to oxytocin impulses. That is, dilation and flow differs between main ducts by 2 to 8 seconds, in the one milk ejection event. The timing of myoepithelial cell response differs, resulting in heterogensous emptying of the gland, and not all myoepithelial cells contract in response to oxytocin.

Selected references

Kent JC, Mitoulas LR, Cregan MD, Geddes DT, Larsson M, Doherty DA, et al. Importance of vacuum for breastmilk expression. Breastfeeding Medicine. 2008;3(1):11-19.

Prime DK, Geddes DT, Hepworth AR, Trengove NJ, Hartmann PE. Comparison of the patterns of milk ejection during repeated breast expression sessions in women. Breastfeeding Medicine. 2011;6(4):183-190.

Ramsay DT, Kent JC, Owens RA, Hartmann PE. Ultrasound Imaging of Milk Ejection in the Breast of Lactating Women. Pediatrics. 2004;113(2):361-367

Ramsay DT, Mitoulas LR, Kent JC, Cregan M. Milk flow rates can be used to identify and investigate milk ejection in women expressing breast milk using an electric pump. Breastfeeding Medicine. 2006;1(1):14-23)

Uvnäs Moberg K, Ekström-Bergström A, Buckley S, Massarotti C, Pajalic Z, Luegmair K, Kotlowska A, Lengler L, Olza I, Grylka-Baeschlin S, Leahy-Warren P, Hadjigeorgiu E, Villarmea S, Dencker A. Maternal plasma levels of oxytocin during breastfeeding-A systematic review. PLoS One. 2020 Aug 5;15(8):e0235806. doi: 10.1371/journal.pone.0235806. PMID: 32756565; PMCID: PMC7406087.