The Australian couple, who have not been named, paid 21-year-old Thai surroate Pattharamon Janbua to carry their babies as they were unable to conceive themselves.
IVF proved successful and the couple were told to expect twins. But when they learned that one of the babies would have Down's syndrome, they asked the surrogate to terminate her pregnancy.
Pattharamon refused on religious grounds, but when the twins were born in a Bangkok hospital, the couple took the non-Down's girl back home to Australia and abandoned her brother, who has been named Gammy.
The case has sparked international indignation after a 'Hope for Gammy' fund-raising campaign was picked up by local media.
Now six months old, Gammy desperately needs a series of operations, which Pattharamon cannot afford, or he will die. Strangers from around the world have already raised more than £35,000.
Young Thai women in need of cash are often used as surrogates by wealthy foreign couples. Pattharamon was promised around £6,500, but said she's still owed £1,300 by the agent who arranged the deal.
She said when she looks at Gammy, she feels sorry for him and guilty. She told the Sydney Morning Herald: "But I think this is not a bad karma ... it's good karma that made us be together.
"I would like to tell Thai women – don't get into this business as a surrogate.
"Don't just think only for money - if something goes wrong no one will help us and the baby will be abandoned from society, then we have to take responsibility for that."
Writing on the fundraising page, many have questioned the actions of Gammy's biological parents.
Michelle Collins wrote: "The biological parents are a disgrace. I wonder if they will abandon the other child if she does not meet their expectations."
Kim Mulrooney added: "His surrogate Mother has called it a blessing, his biological parents don't deserve him. Little fella is going to have a better life with a family who loves him."
Lia Ming said: "It's hard not to judge these parents. But so disgusted with their act. Gammy is still their own flesh and blood despite using a surrogate mother. "
More on Parentdish: Why is Down's syndrome seen as so undesirable by our society?