Mother’s own milk (MoM) is clearly preferable for the enteral nutrition of all infants, with major advantages demonstrated among the preterm. Despite one bizarre, flawed, and seriously biased article, that I have criticized on this blog, the next best substrate is, also very clearly, donor human milk (DHM).

Most DHM around the world is provided by altruistic volunteers who provide milk to their local milk bank after expression into sterile containers. But how do they actually do it? I scanned the on-line information provided by several milk banks, and there was little instruction to prospective donors about how exactly to express their milk.

The Ontario milk bank has a video which suggests pumping 1, 2 or 3 times a day, or pumping when first waking in the morning if the mother wakes with breast fullness, or pumping from one breast while their infant suckles from the other, if the baby seems to have a preference. Information for bereaved mothers suggests pumping for 15-20 minutes every 3 to 4 hours.

The Vancouver milk bank recommends pumping once a day, but in their video they also mention pumping from a contralateral breast, or after feeding the mother’s own baby. A few organisations (such as NICE in the UK) counsel against the use of “drip” milk, that is, milk that is spontaneously secreted from the contralateral breast during a breast feed (such drip milk has been shown to have much lower macronutrient content, especially low fat). Most have no other specific instructions. NICE also encourages hand expression, rather than using a pump; this is not explained. In the document detailing the evidence supporting the guideline, they don’t really explain why manual expression is preferred over an electric pump.

Composition of breast milk changes with gestational age at delivery, postnatal age (Mimouni FB, et al. Preterm Human Milk Macronutrient and Energy Composition: A Systematic Review and Meta-Analysis. Clin Perinatol. 2017;44(1):165-72), and even time of day (a relatively minor effect, Paulaviciene IJ, et al. Circadian changes in the composition of human milk macronutrients depending on pregnancy duration: a cross-sectional study. Int Breastfeed J. 2020;15(1):49). Importantly though, composition changes with timing during a feed (with foremilk having lower, and hindmilk higher, fat content, Bishara R, et al. Nutrient composition of hindmilk produced by mothers of very low birth weight infants born at less than 28 weeks’ gestation. J Hum Lact. 2008;24(2):159-67), As for protein content, published research is somewhat contradictory, some showing that hindmilk also has more protein than foremilk, while others seem to show them as being similar. There are some minor problems with the literature, mostly that the definition of when foremilk becomes hindmilk is unclear, it is usually defined after a specific time interval.

Since the development of the human milk bank in Québec, which is organised by the blood transfusion service (Héma-Québec), the program has had stringent quality control, follows HMBANA guidelines, and has been evaluating the macronutrient content of the milk provided. Donor milk received by our milk bank varies in calorie density between 58 and 72 kcal/100mL, and a typical pooled donor milk sample shows a total calorie content of well below 67 kcal/100mL (the classic 20 kcal/fl.ounce). Thus what we give as DHM usually averages close to 60 kcal/100 mL (18 kcal/oz) so we routinely calculate our donor breast milk fortification on this basis, adding 50% more fortifier to DHM than to MoM to achieve our standard 81 kcal/100ml (24 kcal/oz).

Our nutrition committee and the Héma-Québec milk bank personnel wondered if it would be possible to provide DHM with higher fat, and therefore calorie density, by informing mothers of the potential advantages of hindmilk, and assisting them to provide it. But it was realized, as a preliminary, that we didn’t actually know how mothers were expressing their milk. We have just published the results of a questionnaire, surveying donors to our bank, asking them exactly how they were expressing their milk, and if they would be prepared to change their practices in order to produce hindmilk DHM with higher fat content. (Girard M, et al. Donor Milk Expression Habits: Can we Favor Hindmilk Banking for Extremely Preterm Infants? Breastfeed Med. 2024) 126 of 170 mothers completed the questionnaire, 57% reported expressing donated milk between breastfeeds; 15% reported simultaneously breastfeeding while expressing from the other breast; 12% reported breastfeeding their baby on each breast, then expressing (in other words, providing hindmilk). Most mother/donors were willing to consider changing practice to donate hindmilk, even though this is somewhat more onerous. It entails stopping the breast feed, then washing the breast, then expressing into the sterilized container. Some mothers thought they might not have enough milk left after breastfeeding, or that they were already exhausted after a breast feed, nevertheless 89% were willing to provide hindmilk at least some of the time, and 2/3 could envisage doing so exclusively.

The comments from many respondents (which you can read in the article) were quite touching, and show the dedication of the donors to providing the best start possible for preterm infants. The next stage is to identify exactly how we will identify, and keep separate during preparation, hindmilk, and to ensure that there are no negative impacts on overall donation volumes. Then proving that nutritional outcomes are improved, if possible, although there are already a couple of published studies that we mention in our article and refer to in our bibliography that show such an effect.

Another potential application of this data is for mothers expressing milk for their own baby, would it be possible to provide MoM with more hindmilk? I am sure many mothers would be willing to do so, if we can figure out the logistics.

I don’t suppose there are many breast milk donor who read this blog, but if there are: “Thank You!”, your selfless act benefits many babies who you will never meet.