Lord Justice Judge said: "She is entitled not to be forced to submit to an invasion of her body against her will, whether her own life or that of her unborn child depends on it."
The 30-year-old woman, identifiable only as Ms S, was detained by a social worker under the Mental Health Act after her GP warned she was refusing treatment for severe pre-eclampsia - a condition which threatened her life and that of her unborn baby, now a healthy two-year-old.
The appeal judges ruled that an unborn child's need for medical aid does not prevail over the right of its mother to refuse treatment. They found a High Court judge had acted wrongly in granting an injunction allowing the hospital to operate on the woman without her consent.
After yesterday's ruling, Richard Stein, the woman's solicitor, said: "The position is now clear for all medical professionals and social workers in the future that women patients can decide what they want in relation to treatment over their births."
Ann Furedi, director of the Birth Control Trust, said: "Pregnant women are not walking wombs but individuals who have the same right as anybody else to refuse medical treatment and reject doctors' advice.
"Usually a pregnant woman and her doctors want the same outcome - but when a conflict arises the woman's decision must prevail. It is her body and her autonomy at stake."
The charity MIND welcomed the ruling and said it knew of at least five operations that had been forcibly carried out in the last two years.
Ms S was a single woman who was 36 weeks pregnant when she went to register with a NHS practice in south London in April 1996. Told by doctors that she had pre-eclampsia, she rejected an induced delivery.
The judge said: "She fully understood the potential risks but rejected the advice. She wanted her baby to be born naturally." The court heard that Ms S wanted to go to Wales to have her baby in a barn.
Social worker Louize Collins wrote at the time that Ms S "acknowledged that she is probably depressed, she has had many difficulties of late with relationships, housing, changing jobs and indeed being pregnant with a child that she says she doesn't want".
Ms Collins successfully applied for an order under Section 2 of the Mental Health Act after Ms S adamantly refused to accept the advice of doctors. The social worker also wrote that Ms S had recorded in writing her "extreme objection to any medical or surgical intervention".
After Ms S had been transferred to St George's Hospital, south London, an application was made to the High Court to dispense with her consent to treatment. The judge who granted the injunction, Mrs Justice Hogg, was told wrongly that Ms S was in labour.
Although Ms S did not physically resist when told she was having the operation, Lord Justice Judge said this was not consent but "submission". He said: "How can a forced invasion of a competent adult's body against her will even for the most laudable of motives ... be ordered without irremediably damaging the principle of self determination?"
The judge, together with Lady Justice Butler-Sloss and Lord Justice Robert Walker, agreed that while pregnancy increases the personal responsibilities of a woman it does not diminish her entitlement to decide whether or not to undergo medical treatment.
Lord Justice Judge said that Miss Collins and the doctors had been motivated by a "genuine desire" to do what was best for Ms S and her baby. The judges said they "admired the courage" of the social worker in her attempts to deal with "an unusual, unreasonable mother-to-be".
The hospitals involved, and Merton council, are expected to appeal to the House of Lords.
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