The woman, named as X for legal reasons, sued the Leeds Teaching Hospitals NHS Trust over the failure to pick up her baby's medical condition on ultrascans.
The boy was born in early 1999 with a "cloacal exstrophy", a severe, permanent condition which causes bladder, bowels and genitalia defects.
Mr Justice Holland ruled in the High Court that doctors at the Leeds General Infirmary had negligently failed to pick up on the abnormalities while the child was still in the womb.
Opted
His mother, in her 30s, is now in line for massive damages to compensate her for the additional costs of bringing up a disabled child.
Mr Justice Holland told the court he had to decide whether the mother would have had an abortion had the defects been discovered – and, if so, whether the hospital was negligent.
"I am satisfied had the correct diagnosis been communicated to X, she would have opted for a termination," said the judge.
Though the boy was loved and the mother was devoted, all the other evidence indicated she would have opted for an abortion.
The judge said the chance of an exomphalos – a more minor condition affecting the bladder – had been flagged up at the woman's local hospital when she was four months pregnant. Despite undergoing four ultrasound scans at LGI, doctors failed to pick up that the boy's bladder was missing altogether.
Mr Justice Holland concluded that, although the failure may have been excused had the scan been a standard one, in this case it was a scan with a focus.
He said it had not been easy to reach his judgment and added that those medics subject to criticism came across as "normally caring, dedicated and skilled" in the witness box.
No date was set for when the level of X's damages will be decided.
The law – which views the birth of every child as "a blessing" – means X will only be compensated for the additional costs of bringing up her child associated with his disabilities.
A spokesman for the Leeds Teaching Hospitals NHS Trust said: "We extend our most sincere sympathies to the lady concerned. This was an extremely complex case and one which was defended by the trust because it raised important clinical questions.
"We accept the judgment passed by Mr Justice Holland and can confirm that, as a result of the issues raised in this case, the foetal medicine unit now routinely uses 'colour doppler flow' testing in cases such as this in order to reduce the risk of failure of detection."
louise.male@ypn.co.uk