Dr Elizabeth Pilling described how she was aware an umbilical venous catheter (UVC) in the abdomen of Cassian Curry was in a "sub-optimal" position near his heart when it was inserted by two junior doctors after he was born at Sheffield's Jessop Wing maternity unit on April 3, last year.
Dr Pilling said she had intended to have it repositioned within 24 hours but first waited because of the dangers of repeatedly handling a baby as premature as Cassian, who was born at 28 weeks and weighed just 1lb 10oz (750g).
The consultant told an inquest on Thursday: "I've been round and round - why did I forget?
"It's impossible to remember why you forget things."
She said: "I can't explain why I didn't do it in that situation, apart from the acuity of the unit."
The inquest in Sheffield has heard how Cassian deteriorated rapidly in the early hours of April 5 and died despite desperate attempts at resuscitation.
The tiny baby died from a cardiac tamponade, which is when fluid builds up in the space around the heart, eventually preventing it from pumping.
Dr Pilling told a coroner on Thursday how she had never treated a cardiac tamponade in 13 years as a consultant and was shocked at Cassian's sudden deterioration because he had been doing well for his size and prematurity.
The consultant told the inquest how she was starting work on her fifth consecutive 12-hour day shift on Easter Sunday when she was reminded by her registrar about the position of the line on an X-ray.
She said she did not think it was an urgent matter but had intended that it should be changed at some time during the day but "it went out of my head".
Cassian's parents, Karolina and James Curry, have expressed their concerns that understaffing over the Easter bank holiday contributed to their son's death.
Asked about staffing levels, Dr Pilling said the unit was properly staff, according to national guidelines and there were no absences.
But she told a coroner it was very busy - with large numbers of admissions and discharges plus complex cases - and this was exacerbated by many of the staff on duty being at the junior end of their scales, meaning she was regularly being asked for her support.
Dr Pilling was asked by Ross Beaton, for Mr and Mrs Curry, whether she was aware some junior members of staff had submitted internal incident reports with concerns about staffing issues that bank holiday.
She replied: "I was aware, as they were coming in and telling me how hard it was, how busy it was. They were asking me if I can do anything to help."
Dr Porus Bustani, the clinical lead of the Jessop Wing neonatal unit, told the inquest that the staffing levels that weekend exceeded the national guidelines by some margin.
But Dr Bustani said the unit ?had reviewed the way its consultants worked so one person did not cover the whole of a bank holiday weekend. And he said he was working towards having two consultants on at weekends.
He added that he had been a consultant for 20 years and had only seen a tamponade caused by a line location once before.
"They are extremely rare but, when they happen, they are catastrophic and that's what happened with Cassian," he told the inquest.
Asked by assistant coroner Abigail Combes whether staff were aware of this danger, Dr Bustani said: "They know. The word tamponade is not taken lightly at all, but they also know how rare they are."
He said: "For this to happen 36 to 40 hours after birth is very quick."
Dr Bustani said there were also dangers when UVCs were repositioned and he was concerned the reaction to this tragedy would mean doctors becoming too eager to make that call, thus creating other difficulties.
Ms Combes has told the inquest she will consider whether negligence contributed to Cassian's death when she concludes the hearing on Friday.
The medical director of Sheffield Teaching Hospitals NHS Foundation Trust, Dr Jennifer Hill, has said the trust is "so very sorry for what happened" to Cassian, admitting there was "human error in terms of the management of Cassian's umbilical venous catheter."
The Care Quality Commission (CQC) has raised concerns about maternity services at the Jessop Wing and rated them as inadequate a month before Cassian was born, confirming this judgment earlier this month.
But Sheffield Teaching Hospitals NHS Foundation Trust has stressed that the neonatal unit did not form part of these maternity inspections and judgments.