MONDAY, April 6, 2015 (HealthDay News) -- Parents who buy breast milk online may not always get 100 percent human milk. A new study found about 10 percent of samples tested were contaminated with cow's milk.

"Human milk may be purposely 'topped off' with cow's milk or infant formula, and this could be harmful to babies receiving the purchased milk if they have cow's milk allergy or intolerance," said lead author Sarah Keim, a researcher at Nationwide Children's Hospital in Columbus, Ohio.

"For other babies, cow's milk is not recommended as a primary source of nutrition because it does not provide complete nutrition for human infants," Keim said.

For the study, published online April 6 in the journal Pediatrics, researchers bought 102 samples of advertised human milk online and tested the DNA in each one. They found that 10 samples contained at least 10 percent of cow's milk.

The study authors noted that earning money for breast milk might motivate sellers to add cow's milk to increase their milk volume, but buyers cannot usually test the milk.

"There really is no way a parent can tell if the milk they buy online is safe and high-quality," Keim said. "Given the risks, it is not a good idea to feed your baby milk you purchase online."

In a previous study, Keim and her colleagues found that breast milk samples bought online contained potentially harmful levels of bacteria and tested positive for a virus called cytomegalovirus.

Kim Updegrove, president of the Human Milk Banking Association of North America, said purchasing milk outside of standardized human milk banks is risky.

"When you have monetary incentive and you have women who may be desperate or may not know any better, you have a risk of getting something along with the milk you are purchasing," she said.

"It is important to applaud parents who want to do what's best for their babies, but to remind them that it's a body fluid, and body fluids can be dangerous," Updegrove said.

Besides cytomegalovirus, breast milk purchased online may contain viruses such as hepatitis B, hepatitis C and HIV, said Dr. Susan Landers, a neonatologist in Austin, Texas. Breast milk donations at regulated milk banks are tested for these viruses, and rejected if they test positive.