If you purchase breast milk online for your infant, you may be getting more than you bargained for in the package. According to a new study published in the journal Pediatrics, ten percent of breast milk samples purchased on the Internet have added cow's milk or baby formula. This could pose health risks to your infant with allergies or expose children to potentially infectious diseases.

Researchers tested 102 samples of breast milk that was for sale online and found that while all the purchases did contain human breast milk, 11 were found to also contain bovine DNA.

For the study, scientists used a molecular test that differentiated between human and cow milk and also detected the level of cow's milk in the liquid. This allowed them to exclude accidental contamination from their findings. Of the 11, one tested for minor or accidental contamination while the others contained enough bovine milk to suggest that it was intentionally added.

"In all honesty, I'm not that surprised. Sellers charge per ounce of milk. Cow's milk is about 100 times cheaper than breast milk you can buy online," study co-author Jesse Kwiek, an associate professor of microbiology at The Ohio State University Wexner Medical Center, said. The team paid between $1 and $4 per ounce.

The purchasing of breast milk has become a growing trend in the last few years. It 2012, there were an estimated 12,000 transactions, but this year that number is expected to grow to 50,000.

"I would not recommend, under any circumstance, for a family to purchase unscreened donor milk online," Dr. Amy Hair, neonatologist and director of the Neonatology Nutrition Program at Texas Children's Hospital, said. Hair, who was not involved with the study, focuses specifically on neonatal nutrition, human milk and growth for infants.

You don't really know what you are buying and purchased milk may contain unknown additives that could prove harmful to your baby.

"The primary risk of this is if a child has a cow's milk allergy, there could be an allergic reaction, but also that the child is not getting all the benefits of human breast milk," Kweik said.

"Breast milk diluted with cow milk or milk you buy at the store may not have the full nutrition content that an infant would need," Hair said. "If they can't get a full amount of breast milk, I would recommend supplementing with formula to meet nutrition needs."

Cow's milk is not recommended for children until they reach one year of age and mothers who can are generally advised to breast feed. If that is not possible, formula can be used for the first year because of a child's nutrition and protein needs.

"The take-home message is you really don't know what you're getting," Kweik said of Internet milk purchases.