The practice has led to a call for the Department of Health to issue more guidance to these mothers, who are acting outside of NHS supervision.
Some experts fear the unregulated practice could spread infection and viruses such as HIV and hepatitis. But advocates argue mothers are making an informed choice.
A Facebook page called Human Milk 4 Human Babies UK facilitates breast milk exchanges between those mothers with surplus breast milk and others who need it.
The BBC reports that likes for the page have increased fivefold to almost 18,000 in the past five years. The BBC article shares the stories of two mums, one of whom was struggling to produce sufficient milk for her baby and another who had a surplus of breastmilk.
Sarah McHugh responded to a plea from Bex Poole who appealed for breastmilk on the Facebook page and the two met up.
The Facebook page offers guidance for anyone considering using it and urges people to discuss medications, alcohol or drug use. It suggests using a health care provider for further testing if worried and asking for copies of results.
It suggests looking into home pasteurisation if worries persist.
However, the BBC reported that consultant neonatologist at Birmingham Women's Hospital, Dr Gemma Holder, had concerns about such informal exchanges of breastmilk because using breastmilk outside medical supervision might risk babies' health.
There are 16 official milk banks across the UK and Republic of Ireland where breastmilk can be donated. Donations are checked in line with NICE guidelines before being sent to sick and preterm babies in hospitals.
The milk is pasteurised before it is frozen, ready for use.
The Food Standards Agency said it does not recommend sharing donor breast milk for safety reasons.
Alison Thewliss, the SNP politician who chairs the all-party parliamentary group for infant feeding, told the BBC that the rest of the UK should follow the Scottish model.
One Milk Bank for Scotland is part of the health service and ensures breast milk can be collected from donors, processed and distributed using a well-developed network.
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