Women’s enthusiasm for using these better pumps may be a positive outcome for infants if it means that infants receive more human milk than they would if their mothers did not use a pump. This outcome is suggested in the Internet postings of women who had such difficulty with breastfeeding that they would have fed formula but were instead able to give their infants their milk with the assistance of a breast pump.23
Women may choose to pump because they perceive that it will permit them to feed their infants their milk for a longer period.18
Also, women’s enthusiasm for pumping and their success in producing more milk than their own infants need may lead them to donate their excess milk to a milk bank, where it could benefit infants who might not otherwise have access to the many positive attributes of human milk. Despite these possibilities, it remains unknown whether infants whose mothers pump their milk actually receive more human milk than they would if their mothers had not chosen to pump their milk. This question should be evaluated carefully.
Conversely, substitution of milk expression for feeding the baby at the breast may be problematic for infants for several reasons. The most serious of these relate to the composition of expressed milk and the way it is fed to the infant. For example, expressed milk may become contaminated in the process of transferring it to the infant,24
or the way it is stored may compromise its nutritional and anti-infective benefits. We discuss these possibilities in turn.
Milk expressed with a pump makes contact with nipple shields and valves during expression, and all expressed milk makes contact with a storage container or a feeding vessel before it is fed to the infant. Each of these items is a potential source of contamination if women pump in unsanitary conditions or if the pump and pump parts are not kept scrupulously clean22,25
(). Some research has shown that bacterial counts are higher in milk expressed with a pump than in milk expressed by hand.25,26
Women put their milk in a wide variety of containers (), some of which are unsuitable for this use and can lead to the leaching of undesirable substances from the container into the milk or the degradation of key milk components during storage.27
Glass is the container least destructive of milk components,28,29
although women may use it infrequently. Research is needed to document the ways women handle and store their expressed milk before it is fed to their infants and how its composition changes during this process.
Women often store their milk in the refrigerator and various kinds of freezers for short or long periods.30
This practice may lead to bacterial growth or degradation of milk components. It has long been known that when breast milk is stored at refrigeration temperatures, its ascorbic acid concentration is reduced,31
as is its overall antioxidant activity.32
When breast milk is stored at temperatures common in home freezers, lipids are hydrolyzed,33
immunological cells are lyzed,28
and antioxidant activity is reduced,32
but antimicrobial proteins are unaffected.34
Moreover, microwave thawing of frozen milk, which mothers do, causes a marked decrease in anti-infective factors in milk.35
As a result, expressed milk may not deliver the same nutritional and anti-infective benefits of milk obtained at the breast. The consequences of these differences for infant health are unknown and warrant investigation.27
Expressed milk is most often fed to infants from bottles, and it is likely that caregivers treat human milk in a bottle the same way that they treat infant formula, that is, to encourage infants to finish the bottle. The IFPS II provides recent evidence to support this assertion; Li et al.36
found that infants fed expressed milk in a bottle early in infancy were more likely to empty the bottle later in infancy than were infants who had been fed only at the breast. This effect did not depend on whether human milk or infant formula was in the bottle. More research is needed to ascertain whether infants fed expressed milk are, on the whole, fed differently and thus grow differently37
from those fed milk at the breast.
A final concern for infant health is clinical and comes from the management of milk expression relative to at-the-breast feeding. Milk changes composition over the course of a single feeding38
and with infant age.39,40
This finding is also true of expressed milk; its fat concentration increases with infant age.41
It is not uncommon for mothers to feed their infants at the breast and then express their remaining milk with a pump. Thus, they feed their infants predominantly fore milk (which is high in carbohydrates) at the breast, and they store hind milk (which is high in fat) to feed to their infants later. Consequently, these infants sometimes develop diarrhea and fail to thrive, a result that is analogous to the situation that occurs when infants are overfed at the breast.42
The frequency of this occurrence has not yet been documented and warrants further study.