The woman's mother won a legal battle with doctors, who said the pregnancy should be terminated, in June.
Specialists now plan to deliver the baby by Caesarean section in the near future.
Bosses at an NHS hospital trust responsible for the woman's care said doctors should be allowed to fit a contraceptive device immediately after that operation, while the woman was under anaesthesia.
But lawyers appointed to represent the woman, and a council social worker who works with her, disagreed.
They said fitting a contraceptive device immediately after a Caesarean section was performed would be expedient.
But they said expediency was not enough to justify such an "interference" with the woman's "bodily autonomy".
They said such a move would be premature and said the woman should have more involvement and control over the decision.
Mr Justice MacDonald on Wednesday decided that fitting a contraceptive device immediately after a Caesarean section would be in the woman's best interests.
The judge had analysed arguments about contraception at a hearing in the Court of Protection, where issues relating to people who lack the mental capacity to make decisions are considered, in London.
He said he would publish a ruling outlining the reasoning behind his decision soon.
Earlier this year, bosses at the hospital trust asked a different Court of Protection judge to let doctors perform an abortion.
Mrs Justice Lieven gave specialists permission to terminate the woman's pregnancy after a hearing in London.
But Court of Appeal judges in London overturned that ruling in June after the woman's mother, a Catholic former midwife, mounted a challenge.
Judges have been told that the woman, who is in her 20s, has the mental age of a child aged between six and nine and cannot make decisions for herself.
They have heard that she has been diagnosed with a "moderately severe" learning disability and a mood disorder.
Judges have been told that the "circumstances" of the woman's pregnancy were "unclear" and that a police investigation had begun.
A barrister who represented trust bosses said it would not be in the woman's best interests to get pregnant again.
Fiona Paterson said the woman was vulnerable and could not be supervised 24 hours a day.
She said fitting a contraceptive device immediately after a Caesarean section would be the safest and most comfortable option for the woman.
The woman had been represented by staff from the office of the Official Solicitor, who help vulnerable people embroiled in litigation.
They had instructed barrister Susanna Rickard to make submissions on the woman's behalf.
Ms Rickard said there a "certain expediency" in fitting a contraceptive device immediately after a Caesarean section operation.
But she said expediency was not a "sufficient reason" and would not justify "the interference" with the woman's "bodily autonomy".
Ms Rickard said a council plan was in place to safeguard the woman under which she would not be left home alone, or unsupervised with a man, or unaccompanied in the community.
She said the chance of the woman being exposed to "further sexual activity" in the foreseeable future was "close to nil".
Judges have ruled that the woman, who lives in the London area, cannot be identified in reports of the case. They also say the NHS hospital trust which asked for a decision, and the council which employs the social worker, cannot be named because publication of their names might create an information jigsaw which could lead to the woman's identity being revealed.
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