The former nurse, who is set to receive up to pounds 500,000 in damages, gave birth to a daughter by emergency Caesarean in November 1989 shortly after being told she had pulmonary hypertension, which is made worse by pregnancy.
Mr Justice Astill ruled that the woman, who he said should not be named, was entitled to damages against Croydon Health Authority in south London after a hospital radiologist failed to correctly interpret a chest X-ray that she had in November 1988, five months before she became pregnant.
The authority accepted that the reading of the X-ray, which was part of the selection process for a new job, was negligent in that the radiologist had passed the woman as fit when the problem should have been detected.
But the authority argued that the woman's pregnancy was not a direct or foreseeable consequence of the negligence.
Mr Justice Astill said, however: "Although she was very much wanted at conception, the child would not have been born had a proper diagnosis been made.
"She became an 'unwanted' child only when the plaintiff knew she could not be a normal mother to her, with a normal life expectancy to see her to independence."
The authority said immediately that it would appeal, and the ruling was quizzically received by some expert medical negligence lawyers.
It is the latest in a line of cases in which an unwanted pregnancy has been treated as an "injury", but the earlier decisions have involved mistakes relating directly to the reproductive process, such as negligent obstetric or gynaecological treatment, or failed sterilisations and vasectomies.
British judges are cautious about extending the limits of what is "reasonably foreseeable" for fear of encouraging a United States-style litigation explosion.
But Mr Justice Astill said: "Pregnancy is likely to have such devastating consequences for a woman suffering from this that it should have been at the forefront of the mind of a competent radiologist."
The case is thought to be the first in which damages will be awarded to cover the cost of bringing up a child who was wanted and planned at conception, but "unwanted" at birth.
The judge said that the woman suffered a pulmonary embolism 37 weeks into her pregnancy and later spent time in hospital away from her newly- born daughter.
Even though she was likely to live 20 years from diagnosis, the woman would never work again because of severe reactive depression and she and her husband were now contemplating divorce.
"I don't accept she would have suffered in the same way if the diagnosis was made at the proper time," the judge said.
Damages covering pain and suffering, loss of earnings, the costs of raising the child and pregnancy expenses will be assessed at a later date unless the decision is successfully challenged on appeal.Reuse content