Women are using Facebook groups such as Human Milk 4 Human Babies and Eats on Feets to share breast milk online.
The groups allow lactating women to post details of milk they have to offer and others to make requests for breast milk in their area.
Jessica Herrick, from Manchester, told BuzzFeed News that she started milksharing through Facebook when her son, now 3 weeks old, was born with a cleft lip and was unable to latch onto both breasts.
"Instead of throwing the milk that I am expressing away, I decided to milkshare instead," she said.
Herrick said her decision to share her milk was "so babies who wouldn't have otherwise had the opportunity, can have the best start in life by getting all the goodness of breast milk".
"I think all babies should get the chance to have breast milk," she said, "but unfortunately they aren't always able to."
While she has only just begun milksharing, the concept is nothing new to her. "My mum told me about how she shared her milk 28 years ago when my twin brothers where born 14 weeks early and in intensive care," she said. "I thought it was amazing that other mothers could help other babies by giving them their breast milk."
Kimberly from Oxford decided to donate her milk using Facebook when her baby was born premature and she was unable to even hold him for the first four weeks.
"I was lucky to have good support in hospital that allowed me to express breast milk, and have the hospital freeze a stock of it," she told BuzzFeed News. "Now that I'm home though, our freezer's only so big, but it's important not to waste it, and there are plenty of mothers who need use of it, so I was happy to donate."
Kimberly said that had she needed it, she would have been more than happy to feed her own baby the donor milk that the hospital had to offer. "It's good to be able to get it if you can't personally provide that for you baby," she said.
While Kimberly said she would personally prefer to accept donor milk from hospital stocks that had been pasteurised and came from women who had had formal blood tests, she added: "I am happy to share my pregnancy blood test results with anybody receiving my donations via Facebook." Sharing milk is a "deeply personal decision", she said, "but if you know and trust that person it's an easier decision to make".
New research by Netmums shows that 1 in 25 women say they have breastfed another woman's baby and 1 in 50 are using Facebook groups to share milk.
They found that a quarter of the 2,000 families surveyed used milksharing sites when a mother was too unwell to breastfeed after giving birth, while 22% experienced low milk supply and 16% had medical conditions that meant they were unable to feed.
The peer-to-peer nature of Facebook groups means that mothers can easily specify whether they want milk from a woman who is a vegan or vegetarian, or to arrange milk from women of a particular religion.
The not-for-profet groups are run by volunteers. A third of the donors surveyed by Netmums said they wanted "to help a family in their hour of need".
Meanwhile, some women are physically breastfeeding other people's children for them. Earlier this month, Jessica Anne Colletti shared a picture on Facebook of herself breastfeeding both her 16-month-old son and a friend's 18-month-old son.
Colletti said she babysits the 18-month-old while her friend works and that she has been breastfeeding them both for a year. She described the boys as "milk siblings", and said there was a "special bond between us all".
The post sparked a mixed reaction. While one commenter said breastfeeding a friend's son was a "selfless gift", and another described the act as the "beautiful chain of motherhood passed down from the very first humans", others weren't quite so positive.
Not everyone thinks that breastfeeding someone else's child is natural, and Kimberly said she was not surprised by negative comments.
She told BuzzFeed News: "We have a thing with breasts in our culture what with Page 3 and everything, so it's no surprise that people feel uncomfortable seeing this.
"A lot of people say they're disgusted by a lot of things. It's brave of women in that situation to ask for and accept help."
Laura Silver is a reporter for BuzzFeed News and is based in London.
Contact Laura Silver at firstname.lastname@example.org.
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