Kerry Robertson, 17, has already been prevented from getting married after social workers said she lacked the capacity to consent to the wedding.
Miss Robertson, of Dunfermline, Fife, is due to have her baby in January.
Fife council said "no decision" had yet been made on the baby's future and Miss Robertson would be given every support.
She is now involved in an ongoing legal dispute to determine whether she has sufficient mental capacity to marry her fiance Mark McDougall.
The couple have been together for almost a year and had been due to get married in September.
Mr McDougall, 25, told BBC Scotland: "No matter what their disability or background, you should have the chance to be a family, to have a family life. That's actually a European law as well."
Miss Robertson has already named her child Ben and the couple plan to fight the decision.
Mr McDougall said: "I know this is all that Kerry wants and it's been upsetting her terribly and she's been crying herself to sleep every night since she's been told.
"I'm not going to stand by and let it happen. I'm not going to stand by and let them destroy my fiancee's life because she's a lovely girl with a heart of gold and she doesn't deserve this."
Stephen Moore, the executive director of Fife Council social work service, said "no decision" had yet been made on the baby's future.
He added: "People, including the family, will make a judgment about the welfare of the baby after the baby is born.
"It will be decided then whether she has the capacity to look after the baby. People will work with the young mother to look after the baby. She will be given every support.
"But someone needs to make the judgment about whether she can care for the baby, and that decision will be taken when the baby has been born.
"Much of the work we do is governed by legislation. Complex decisions are made that balance risk and welfare while supporting people at times of personal or family need."
Norman Dunning, chief executive of Enable Scotland, said: "We would be very concerned if her learning disability was perceived as a barrier itself to her marrying and bringing up her child.
"People with learning disabilities do not have loving disabilities.
"There is both research and practice evidence that people with learning disabilities are no worse at bringing up children than other families facing disadvantages - what is crucial is that they get the right level of support from the start."
Mencap's chief executive Mark Goldring said there were about 250,000 parents with a learning disability in the UK and argued all of them should have the same opportunities as anyone else to be a parent.
He said: "With the right support people with a learning disability can be - and are - excellent parents, yet many fear that asking for help may be seen as an admission of failure which would result in their children being taken away."