Last week, a judge at the Court of Protection gave the go ahead for a termination despite the woman's mother and social worker pleading for the pregnancy to continue.
Specialists at the NHS said abortion was the best option.
The judge said some evidence in the case was "heartbreaking".
Mrs Justice Lieven heard that the woman, who is in her twenties and is 22 weeks pregnant, had the mental age of a child between six and nine.
She said the woman did not have the mental capacity to make the decision, although evidence indicated that she wanted to keep the baby.
Bishop John Sherrington, who speaks on behalf of the Church on life issues, said the ruling infringes the human rights of both the mother and the unborn child.
He said: "Forcing a woman to have an abortion against her will, and that of her close family, infringes her human rights, not to mention the right of her unborn child to life in a family that has committed to caring for this child. In a free society like ours there is a delicate balance between the rights of the individual and the powers of the state.
"This is a sad and distressing decision for the whole family whom we keep in our prayers. This case, for which all the information is not available, raises serious questions about the meaning of 'best interests' when a patient lacks mental capacity and is subject to the court's decision against her will."
During last week's hearing, the judge was told that the "circumstances of the conception" were "unclear" and a police investigation was ongoing.
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