A WOMAN posing as a nurse abducted a three-day-old baby from her crib in a hospital ward yesterday, just hours before the newborn was to be taken home. A ward assistant who came face-to-face with the woman on a staircase did not realise what was happening and let her go. Police searched the Caritas Medical Centre in Cheung Sha Wan and the nearby hillside for several hours but found no trace of the baby girl. The Hospital Authority, expressing deep concern, has promised an internal investigation in co-operation with Caritas Chief Executive Ms Helen Tinsley. The woman who took the baby from the hospital's post-natal ward was wearing a white dress, but no nurse cap or hospital name tag. She had claimed that she had to collect the baby for a doctor's examination at about 11 am - which was outside visiting hours and shortly after the mother, Mrs Leung Wong Yuk-ying, had gone to a briefing on child care. The patient in the next bed said the woman, aged about 20, went to Mrs Leung's daughter, which was in the cot closest to the exit. The woman asked where the mother was and then took the baby, saying that ''the doctor would like to conduct some checks''. Mrs Leung, 32, was a few metres away in another corner of the ward attending a nurse's briefing with nine other patients, who were due to be discharged in the afternoon. Her 34-year-old husband, Mr Leung Wing-tak, was expected to arrive at about that time to take his wife and daughter home to Tsuen Wan. The couple have two other daughters. Mrs Leung returned to her bed after the briefing but did not realise the baby had been abducted until about an hour later when she asked a nurse about the ''doctor's examination'' and was told nothing had been arranged. A waitress at a nearby restaurant reported that a woman in a white dress and carrying a baby had gone into the toilet shortly after noon and come out wearing a different dress. Mr Leung last night was still helping police with investigations and declined to comment. Psychologists could offer no firm reason why the woman took the baby. One senior psychologist said: ''It is very difficult to say why this woman did this thing. There are numerous possibilities ranging from the loss of a child, to mental problems or another form of trauma.'' Ward superintendent, Dr Robert Chin Kien-howe said that all nine nurses on duty in the ward at the time were busy looking after 25 patients there, but none was assigned to sit guarding the ward exit or to check people's movements. But he said doctors would visit the ward themselves and never send nurses to take a baby away from its mother for examination, and that anyone would need authorisation to remove a baby from the ward. Extra security guards were deployed outside the post-natal ward after the incident, but Dr Chin defended the policy of allowing babies aged above one day to stay with mothers and of encouraging frequent visits by families. ''We feel very sad about the incident, which is the first in almost 30 years of our history. We will review the policy to prevent any further unauthorised removal of a baby, but the open policy shall be maintained,'' he said. The Hospital Authority said: ''The hospital environment is a very open one, and there are many visitors each day, especially during visiting hours.'' Mr Raymond Wong Siu-kung, general manager, administrative services, at Queen Mary Hospital, said babies were safe at his hospital. ''Insofar as Queen Mary is concerned, I can't think that under normal circumstances this could happen. It shouldn't happen,'' he said. ''All the staff in the hospital, whatever discipline they may belong to, have to wear a permanent staff card'' listing rank and grade with a photo, he said. Parents, too, must show their personal identification cards before they can take their baby out of the hospital, he said. And strangers walking in the paediatric ward would be questioned by staff. he added. Security measures were particularly tight at the paediatric ward because the youngsters there were seen as vulnerable, he said.