infant birth weight and frequency and duration of feeding during the first 14 days post partum.
The interaction of gestational age and birth weight may have a stronger influence on milk intake than does either one alone, because preterm infants (especially those born at <34 weeks of gestation) may be too weak or immature to suck effectively. Studies of the volume of milk produced by mothers of preterm infants are complicated by the fact that many mothers must pump milk for several days or weeks before the infant can suck directly from the breast. The degree of maternal motivation to breastfeed plays a large role in the success of this phase.
Self-regulation of milk intake was studied among 18 exclusively breastfed infants of mothers who increased their milk supply by expressing extra milk daily for 2 weeks (Dewey and Lönnerdal, 1986). On average, these infants took in more milk immediately following this 2-week period, but about half of them returned to near baseline levels of milk intake after another 1 to 2 weeks. Net change in milk intake at the end of the study was greater among heavier infants and was not associated with baseline milk volume. This indicates that milk intake was influenced more by infant demand than by maternal capacity for milk production. In a subsequent study, Dewey et al. (in press) showed that residual milk volume (the difference between the amount that can be extracted by pump compared with usual infant intake) averages about 100 g/day, even among mothers whose infants consume relatively low amounts of milk (<650 g/day). Likewise, Woolridge and Baum (1988) demonstrated that when 29 mothers randomly selected the breast from which to feed the baby first, intake from the second breast was only about 60% of the amount taken from the first breast. These results illustrate that infants ordinarily do not take all the available milk and therefore govern their own intake to a considerable extent.
Self-regulation of milk intake by infants was also demonstrated by Stuff and Nichols (1989), who studied 45 breastfed infants before and after they began consuming solid foods. Energy intake per kilogram of body weight of these infants during exclusive breastfeeding was considerably lower than the Recommended Dietary Allowance (NRC, 1989) and did not increase after solid foods were introduced. Instead, the infants responded to solid foods by reducing breast milk intake, thereby maintaining constant levels of energy intake. Similarly, Nommsen and colleagues (1989) found that solid foods displaced energy intake from human milk in 6-month-old infants even though they were breastfed on demand.