Stefani Tatavitto, 23, regularly feeds both her own two-year-old son Gage and her friend Chrystal Klein’s 19-month-old daughter while Klein works at a factory.
When Tatavitto goes on a night out, Klein, also 23, returns the favour, nursing both children as if they were her own.
Tatavitto, who also has a four-year-old son, Noah, said: "I don’t think it’s shocking to nurse another woman’s child – breastfeeding is natural."
The friends, from Connecticut, America, – who met on Facebook five years ago – started their arrangement when Klein’s daughter, Ariana, was ten months old.
The women said neither of their partners are fazed by the wet nursing and although Tatavitto nurses Ariana in public, passersby are oblivious to the fact that the child is not her own.
Tatavitto breastfeeds Ariana up to six times a week, but Klein insists she isn’t worried about her friend forming a stronger bond with a daughter than she has.
Tatavitto said: "I think it helps Ariana, it keeps her calm and helps her when she’s teething. It’s a big comfort and helps keep her settled.
"If anyone has a problem with it I have not nice words to say to them.
"You drink breast milk from animals, it’s not that weird to drink breast milk from another human.
"I don’t care what anyone else has to say – we do what's best for our families."
Klein, who also has a five-year-old-son, Aiden, says that she is happy for her friend to continue feeding her daughter until Ariana weans.
She said: "Because of Stefani wet nursing I don’t need to pump milk for my daughter or worry about her not having any milk.
"It also makes me feel better to know that my daughter still has that comfort even while I am away from her.
"I am not worried about my daughter becoming too attached to Stefani. Whenever we are both with her and she wants milk, typically she will ask me to nurse.
"There are no drawbacks. I only see positives from this."
So far Tatavitto and Klein haven’t received any criticism for their arrangement, but said they realise some will regard cross-feeding as controversial.
Klein said: "My family and friends don’t know that much about our wet nursing arrangements.
"When it comes to breastfeeding though my family is supportive with every decision I make.
"Some people might question it, because I know it’s not ordinary to wet nurse nowadays."
Breastfeeding can spread infections, such as hepatitis and HIV, but the two friends trust each other and what they put in their bodies.
Klein said: "My partner was sceptical because she's unsure about what Stefani is putting into her body. Not Stefani specifically but just anyone wet nursing Ariana.
"But we know Stefani has a pretty good diet and she doesn’t smoke or drink.
"We were both pregnant at the same time and we were tested for infections and both of us were clear.
"The fact that Ariana is getting milk from the tap, I mean that’s exactly how we should drink it."