Putting a gravely-ill baby on a ventilator if her condition gets worse would not be in her best interests, the High Court was told on Monday.
Charlotte is brain-damaged and needs an oxygen mask
A paediatrician said 17-month-old Charlotte Wyatt's condition had improved since a ruling in October that she should not be resuscitated.
But the doctor said ventilation would still not be the right action to take.
Darren and Debbie Wyatt, of Portsmouth, Hampshire, are challenging the previous ruling that she should not be saved.
Back in October, doctors won the legal right not to resuscitate Charlotte after arguing that she was brain-damaged and "had no feelings other than continuing pain".
Charlotte was born three months premature at St Mary's Hospital in Portsmouth in October 2003, with serious brain, lung and kidney damage.
Now the High Court, sitting in Cardiff, is hearing evidence from experts saying she can now see, hear, smile and enjoy being cuddled.
But when asked about putting Charlotte on ventilation, the paediatrician, referred to in court as Dr H, said: "I feel that it wouldn't be in her best interests - with the severity of her chronic lung disease, an acute respiratory infection is what would be fatal to her. To ventilate would only postpone it.
"I cannot really envisage a situation where it would be in her interests."
She admitted Charlotte's condition had improved since the New Year.
"Her general condition improved, she was more settled, spending more time awake but not in distress, and not requiring as much sedation.
"She does make facial movements but I have never seen her smile, and I have held her, and talked to her and engaged with her but I've never seen her smile."
David Wolfe, representing Charlotte's parents Darren and Debbie, put to the doctor that it was "quite a dramatic change" from the situation described back in October.
Dr H replied: "Yes. Really the only factor is that she is no longer in distress all the time and needing sedation."
The case, being heard by Mr Justice Hedley, is expected to last two days.