A judge on Friday concluded that a pregnancy termination was in the woman's best interests.
Mrs Justice Lieven had analysed evidence 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.
But three appeal judges on Monday over-ruled that decision after the woman's mother, a former midwife, mounted a challenge.
Lord Justice McCombe, Lady Justice King and Lord Justice Peter Jackson had considered the challenge at a Court of Appeal hearing in London.
They said they would give reasons for their decision at a later date.
Lawyers said they thought the circumstances of the case were unique.
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