The practice of buying breast milk online has gained significant traction in recent years, as more parents who can’t produce enough breast milk on their own turn to nursing mothers on the Internet to fill the need. But some breast milk purchased online might be topped off with cow’s milk, which could cause allergic reactions or other problems for some infants, according to a new study published in the journal Pediatrics. The study found that 10 percent of samples purchased online tested positive for bovine DNA and contained at least 10 percent cow’s milk. In other words, one out of 10 bottles of breast milk purchased online could be diluted with cow’s milk. Testing 102 bottles of milk purchased online, the researchers found 11 contaminated with bovine DNA, with 10 of those containing cow’s milk at 10 percent or more by volume. The researchers say some nursing mothers may be economically motivated to mix cow’s milk in with breast milk, which can fetch up to $2 per ounce from online marketplaces. In 2013, the same researchers found that the majority of milk purchased online “exhibited high overall bacterial growth and frequent contamination with pathogenic bacteria, reflecting poor collection, storage, or shipping practices.” In 2010, the U.S. Food and Drug Administration began warning parents against purchasing breast milk that has not been tested, as it may contain contaminants or otherwise pose a health risk to children.