"Based on the estimates obtained here, if women in developed countries had 2·5 children, on average, but breastfed each child for 6 months longer than they currently do, about 25,000 (5%) breast cancers would be prevented each year, and if each child were breastfed for an additional 12 months about 50,000 (11%) breast cancers might be prevented annually."

(Source: Collaborative Group on Hormonal Factors in Breast Cancer, "Breast cancer and breastfeeding: collaborative reanalysis of individual data from 47 epidemiological studies in 30 countries, including 50,302 women with breast cancer and 96,973 women without the disease," The Lancet, Volume 360, Number 9328 [20 July 2002].)

"'The risks go down the more children you have,' Ms. Beral said. 'Even if they'd never breast-fed, the risk of breast cancer went down by 7 percent for every additional child.'"

(Source: Prof Valerie Beral, Cancer Researcher at Oxford University, in The Associated Press, "Breast-Feeding Again Linked to Less Cancer," The New York Times, July 23, 2002.)

"A number of studies have suggested a positive association between breastfeeding and cognitive and intellectual development in early and middle childhood."  Thus commences a new study conducted by Danish and American researchers, and published in the Journal of the American Medical Association.  However, this study seeks to examine the association between the duration of breastfeeding and young adult intelligence.

Using data included in the Copenhagen Perinatal Cohort, 9125 people born at Copenhagen University Hospital between October 1959 and December 1961, the researchers were able to determine the duration of breastfeeding, and then to perform two intelligence tests upon two separate (nonoverlapping) samples from within the cohort.

The results show a significant positive relationship between the duration of breastfeeding and IQ score: "Although covariate adjustment somewhat reduced the effects associated with duration of breastfeeding, the differences between the less than 1 month and the 7- to 9- months category remained substantial. The unadjusted and adjusted differences in mean Full Scale IQ between these 2 breastfeeding categories were 10.1 and 6.6, respectively."  The Wechsler Adult Intelligence Scale (WAIS) Full Scale IQ shows an increase in the adjusted mean IQ from 99.4 for those adults who were breastfed for one month or less as infants, to 101.7 for two to three months breastfed, 102.3 for four to six months, and 106.0 for seven to nine months (p=0.003). 

The authors note, "Both the unadjusted and adjusted IQ means showed a dose-response relationship with duration of breastfeeding up to 9 months, but a lower mean IQ in the more than 9-months category," although the difference between the 7 to 9 moths category and the over 9 months category was not statistically significant. In other words, "no additional positive effects are associated with breastfeeding after 9 months."

The Danish and American researchers describe three possible explanations for the "observed positive association between breastfeeding and cognitive development: (1) differences in human milk and infant formula...(2) factors associated with the feeding situation, i.e., physical and psychological contact between mother and child; and (3) unidentified factors...that are associated with the choice of feeding method."  As the authors point out, it seems obvious that the physical and emotional bonding between mother and child fostered by breastfeeding has a positive effect upon the child's cognitive development during his first year, but does not seem to explain the long-term effects into adulthood.  "Perhaps," the authors conclude, "a more viable hypothesis is to consider duration of breastfeeding as an indicator of the interest, time, and energy that the mother is able to invest in the child during the whole upbringing period."  That is, "it may be that mothers who spend more time breastfeeding during the first year of life also spend more time later interacting with the child."  Whatever the mechanism, the results are clear: breastfed babies become more intelligent adults.

(Source: Erik Lykke Mortensen, Kim Fleischer Michaelsen, Stephanie A. Sanders, and June Machover Reinisch, "The Association Between Duration of Breastfeeding and Adult Intelligence, JAMA, vol. 287, no. 18 [May 8, 2002], p. 2365-2371.)