Giving evidence in court was an "ordeal" for which he had no formal training and often left his head spinning, Sir Roy told a disciplinary panel of the General Medical Council (GMC).
He is facing one charge of serious professional misconduct regarding evidence he gave at the trial of Sally Clark, the solicitor convicted then cleared of murdering her two baby sons.
Mrs Clark was found guilty of the murders in 1999, after Sir Roy told the jury that the odds of two babies in the same affluent family falling victim to cot death were one in 73 million.
Other women, including Angela Cannings, from Cornwall, and Donna Anthony, from Somerset, were jailed after trials that relied on the one in 73 million figure put forward by Sir Roy, one of the world's most respected child abuse experts.
Mrs Clark was freed on appeal in 2001 after it emerged that the real risk was around one in 8,500 families.
The GMC hearing has been told that the original statistic was "flawed and misleading" and that Sir Roy's evidence to the trial had been incompetent.
If found guilty, he could be struck off the medical register.
Giving evidence in his defence yesterday, Sir Roy said criminal trials were like "legal jousting matches" which prevented expert witnesses from giving juries the full picture because they could only answer the specific question they were asked.
"Giving evidence in court is a great ordeal, and anyone who gives evidence in court is very nervous, however many times you have been there," he said. "You do tell the truth and nothing but the truth, but there is a falsity of saying you are telling the whole truth as a paediatrician. This is what you have to do and it is very unsettling. You go into the witness box with your mind spinning sometimes".
Doctors did not like having to assign blame and be responsible for "retribution, vengeance and punishment", Sir Roy said.
"It is an uncomfortable area and I consciously run away from it," he said.
Sir Roy was the paediatrician who first identified the condition Munchausen Syndrome by Proxy, where people deliberately harm or kill someone else in order to gain attention. He went on to give expert evidence in scores of care proceedings and criminal trials involving parents accused of abusing their children, while many other cases also relied on the "Meadow's Law" of one in 73 million for prosecutions.
Sir Roy said that he had been approached by the police and asked to review the medical records of Mrs Clark's sons Christopher and Harry after she was arrested in 1998. He said that, at the time he believed that there was evidence that both the boys had been smothered. "I did not know what combination could be responsible for the injuries that were described but I could not think of natural causes for the two deaths," he said.
Prior to his appearance yesterday Sir Roy was backed by the medical journal The Lancet, which said that he should never have been charged by the GMC. Richard Horton, editor of The Lancet, said that the paediatrician had been made a scapegoat.
Mrs Clark and her husband Steve said they were "incensed" by the remarks and said the GMC had protected Sir Roy for too long.
They said: "The continued recital of mistruths about our family and the deaths of our babies seems never ending, it hinders Sally's recovery from the miscarriage of justice she suffered and increases the amount of the likely compensation which will be paid to her by the long-suffering British taxpayer."