The number of women buying, selling and sharing breast milk is growing rapidly. But it can be a risky purchase, scientists say, because a mom can't tell by looking at the milk whether it's safe and nutritious for her baby. iStockphoto hide caption

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A lab technician at the Mothers' Milk Bank Northeast in Newton Upper Falls, Mass., prepares donated breast milk for pasteurization in August 2012. The process kills harmful bacteria. Elise Amendola/Associated Press hide caption

itoggle caption Elise Amendola/Associated Press