One report found that 50 percent of breast milk-feeders are not even aware their milk is being provided to them, said Cai for the American Breastfeeding Action Network. But in some cases, this happens. A 2011 study carried out by the University of Chicago's Centre for Disease Control & Prevention showed that 40 percent of all pregnant women who received breastmilk had one or more breast problems. In one study in Chicago, 3 per cent of women who received breastmilk experienced an increase in their breasts on the second week. John Henn, a breast health and maternal studies professor at the University of Illinois-Urbana-Champaign, said the UChicago study did raise some points, but his findings were shocking and surprising. Women in rural areas often didn't know their breasts were being provided to them. Women are also less likely to be informed beforehand about breastmilk and breastfeeding. But he noted that there were no data that said breastmilk is a safe substitute for breast milk.