By Nick Triggle
Health reporter, BBC News
If Pearl Pope could have held her daughter when she was born, she would have fitted in her hand.
Heather was given a 10% chance of surviving
Heather weighed less than 1.5lbs when she was arrived early after just 23 weeks of pregnancy in 2002.
Doctors gave her a one in 10 chance of surviving. But she was lucky - if she had been born 20 years ago she would have had no chance.
It is only recently that those born so prematurely have been able to survive.
Babies such as Heather have benefited from major improvements in neonatal care.
Doctors say they have become better at ventilating and controlling babies' temperatures.
Women who are likely to give birth early are also given steroids to help develop the babies' lungs - the last things to properly form before birth.
There is even talk of an artificial womb being developed which could push back the boundaries even further.
But these advances come at a price.
At 23 weeks, only one in 10 survive, with many going on to live with severe disabilities, raising questions over whether doctors should intervene in the first place.
The Nuffield Council on Bioethics has set up a group of experts to look into the issue. They have reported that babies born at or before 22 weeks should not be resuscitated or given intensive care.
The law currently allows doctors and parents to withdraw treatment if they feel it is in the best interests of the baby.
Although some feel there is an argument for going even further. The Royal College of Obstetricians and Gynaecologists has suggested there needs to be a debate about whether doctors should be allowed to kill severely disabled babies as happens in countries such as Holland.
The college has been supported by Professor John Harris, a bioethics expert at Manchester University, who said the result of withdrawing treatment was the same as actively intervening to end life.
"Does it make a difference if it is brought about by more active means?"
But Ms Pope, 37, from Cumbria, believes it would be wrong to take such a stance.
She said Heather, who has just started school, now shows no physical or mental problems.
"The doctors initially told us they would not do anything, but we insisted they at least try, and thankfully they came round.
"Heather spent 10 weeks on a life support machine which wrecked her lungs and then she was on oxygen for six months, but she is like any four-year-old now - running around, chatting away.
"I can see there are cases where treatment might not be appropriate - if it was just about prolonging the end.
Pearl Pope cuddles her daughter for the first time
"When Heather was three days old doctors said her blood pressure was low and her organs could starting packing up. I would have been okay with it then, but in the end I think parents need to be making the decisions with the advice of doctors."
Vivienne Nathanson, head of ethics at the British Medical Association, also believes so-called mercy killing would be a step too far.
"The risk is that it will change the doctor / patient relationship if we allow mercy killings.
"There are cases where there is no prospect of survival and where treatment just prolongs the agony where it maybe appropriate to withdraw treatment but that is a decision for clinicians and doctors to come to on a case-by-case basis."
But one of the problems is that doctors and parents do not always agree.
Portsmouth's St Mary's Hospital and the parents of Charlotte Wyatt have fought several legal battles over her treatment after doctors said she should be allowed to die.
Charlotte was born three months prematurely in 2003 with serious lung and brain problems, but has now confounded medical opinion by getting past her third birthday.
David Field, professor neo-natal medicine at the University of Leicester, said cases such as this are rare.
"I think it is important that we have a debate over these issues. I think doctors are pretty comfortable with the law as it stands.
"But if anything, I think there is a problem with the expectations of the public. Sometimes they think all it is putting a baby in an incubator and they will be okay.
"But it is much more than that. Intensive care can be very draining for all concerned and then there is the prospect of severe disabilities later in life.
"And it is not just premature babies either, babies that have been starved of oxygen or have congenital abnormalities can also be affected."
And finally there is the issue of money, although it is unlikely to be addressed by the Nuffield Council.
Intensive care costs £1,000 a day on average and after that there may well be rehabilitation and caring costs for a health service already mired over £500m in deficit.
Daniel Sokol, a medical ethicist from Imperial College London, said: "It is a taboo subject. You will never hear it mentioned around the committee table, but you cannot escape the fact that we are talking about expensive procedures.
"But on top of all the difficult ethical questions, the issue of finances cannot be ignored."