Every hospital has a number of small, sparsely furnished rooms used by doctors to impart bad news, with a sofa, soft lamp and box of tissues on the table.

In the past six months, Amy and Nick Brace, parents of a 21-week-old baby girl called Marnie, have been ushered into these rooms with grim regularity.

The first was before their beloved daughter was even born, and then most recently, just six weeks ago, to be told she is desperately ill and will die unless she is given a new heart.

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In the past six months, Amy and Nick Brace, parents of a 21-week-old baby girl called Marnie, have been ushered into rooms used by doctors to impart bad news with grim regularity. Above, Amy with Marnie

'You know when they guide you towards those little rooms that something very serious is happening,' says Amy. 'It's never going to be good news. It feels like we've been in and out of those rooms forever.'

Marnie made headlines this week as the youngest patient currently on the UK's heart transplant waiting list.

She was born with several severe heart defects, including a condition called hypertrophic cardiomyopathy, where the walls of the heart are much thicker than normal and seriously impair its function.

In Marnie's case, this may be a result of a genetic disorder called Noonan syndrome — little-known despite affecting one in every 2,500 liveborn babies in the UK — which is sometimes hard to detect until cardiac problems appear.

Marnie (pictured with her parents) was born with several severe heart defects, including a condition called hypertrophic cardiomyopathy, where the walls of the heart are much thicker than normal and seriously impair its function

Marnie doesn't have the facial features characteristic of the disorder (wide-spaced eyes and low-set ears, for example), and genetic testing hasn't yet given a positive result, but a full diagnosis is ongoing.

And yet as she lies on a pink bedspread in her cot at the Evelina Children's Hospital in Central London, she is alert and obviously happy, following the movements of her 21-month-old brother, Jonah, with big blue eyes and smiling up at the nurse who changes the feeding bag hooked above her.

'She idolises her brother,' says Amy. 'And she's not a laid-back baby; she's quite cheeky. If she doesn't want to do something, she won't do it.

'The fact is we're on untrodden territory here. She's on experimental levels of heart medication, which is scary, and the doctors are constantly monitoring her just to keep her stable. We're living one day at a time, but what she needs is a new heart as soon as possible. She just has to hold on until it can happen.'

Their pain is compounded by the terrible knowledge that child transplant patients are not expected to live to late adulthood. Indeed, average life expectancy after receiving a new heart is only 15 to 20 years.

Some people have had more than one heart transplant, however, and in theory it may be possible to extend Marnie's life with successive transplants. For now, these thoughts seem a long way off — and what parent could put such statistics above their own baby's survival?

Marnie doesn't have the facial features characteristic of the disorder (wide-spaced eyes and low-set ears, for example), and genetic testing hasn't yet given a positive result, but a full diagnosis is ongoing

This is a dilemma they never imagined they'd face — who does? — and they're both acutely aware that what Marnie needs is also the most difficult thing for another family to give. 'What it comes down to,' says Nick, 'is that we're waiting for another child to die.'

'There's no word for what we're asking,' adds Amy. 'Gift doesn't cover it, does it? And I know it's a terrible thing to ask, too, and that just because I'm going through this doesn't mean I have the right to expect it of someone. All we want is for people to give the concept of organ donation some thought.

'We know, more than anyone, that when you have a new baby you think only of the wonderful things your child has ahead of them.

'Most people have no idea where they really stand on organ donation, and if something horrific does happen, in the middle of all that grief and shock, the last thing you can focus on is the person who comes into the room and asks about donating your child's organs.

'I know I wouldn't have been able to, before this. But then the chance to save other children is missed. All we want is that people know their own minds now, just in case.'

While calm and eloquent on these issues, it's clear that deep sorrow lies not far beneath for this couple.

'Some days Nick can't stop sobbing,' says Amy at one point, as she watches him take Jonah for a walk around the hospital canteen.

'We work well as a team, but it's a see-saw — one of us will be relatively OK and the other will be low, and then the next day, we've swapped. But not coping isn't an option.'

They take turns to spend time by Marnie's bedside so that she's never alone. Nervous exhaustion is Amy's constant companion. 'If I'm honest, I was scared just by the prospect of being a mother of two at the age of 30,' she says.

Marnie's parents' pain is compounded by the terrible knowledge that child transplant patients are not expected to live to late adulthood. Indeed, average life expectancy after receiving a new heart is only 15 to 20 years. Also pictured, Marnie's brother, Jonah

'But I'd always wanted my children close together. I'm very close to my own two sisters and I wanted them to have that kind of relationship, too. And now… what I'd give for all the mundane bits of motherhood. Those normal, every-day chores.'

To break up a squabble. To tread on Lego with bare feet. To collapse exhausted at the end of the day, purely from the exertion of being a full-time mother. To not spend every waking moment hoping, somewhere, someone else's child might die, so your child can live.

A year ago, with their second child on the way, the family moved from Milton Keynes in Buckinghamshire to Eastbourne in East Sussex, and Amy — a civilian police caseworker — started putting down roots, making friends, joining playgroups and thinking about schools.

Meanwhile, Nick, 42, did up the new house and threw himself into his recruitment business.

The future was looking good, with its promise of busy motherhood and a new working life for Nick.

Most people think getting a new heart will fix a child for ever, and it's what we thought, too. But it doesn't Marnie's mother, Amy

Then, 31 weeks into the pregnancy, Amy began to worry that the baby wasn't moving enough inside and that her bump was too small.

She was sent to hospital in Hastings for a check-up scan. Not long after, she and Nick were led into the first of those foreboding little rooms.

'The sonographer said the walls of her heart looked a bit thick and that we needed a second opinion. Everything moved very fast after that.'

Referred to the Evelina, which is attached to St Thomas' Hospital, Amy and Nick spent hours waiting for a tentative verdict.

When it came, it could hardly have been worse: the defects in Marnie's heart were severe and it was possible she might not live.

'I think that was probably the worst time of all,' says Amy.

'We just fell apart. I went home thinking, well, it's up to me to keep her alive. It felt like such a burden, such a responsibility. I was constantly checking her movement, constantly on tenterhooks, sending myself crazy. We both fell into a kind of depression at that point.'

At 37 weeks, in May this year, Marnie was induced at the Evelina and, defying expectation, emerged 'in good shape' — a small but chubby 6 lbs 10 oz with a full head of hair.

At that point doctors believed she could live with her own heart, and she was sent home at ten days old on stabilising drugs. And yet her survival was far from certain. 'We did a first-aid resuscitation course in the maternity ward at the Evelina and I cried the whole way through,' says Amy.

'The reality was we were going home with a very fragile baby who could die at any moment. All those things people say about babies — oh, they're tough, they're resilient — none of that was true of Marnie.'

For a few 'blissful' weeks, they were a family together, conscious of making the most of that summer.

It didn't last. Breast-fed at the start, Marnie began to have trouble feeding and required a tube inserted through her nose straight into her stomach, which Amy and Nick had to sterilise and manage at home.

'It was terrifying. The one thing she needed to do was to grow, because that would help the heart condition, but instead she was screaming whenever we tried to feed her until her face went grey.'

Instinctively, Amy knew her baby's condition was deteriorating, and yet she was so tired she found it hard to trust her judgment.

'I remember saying to Nick, 'Is it me, or does she look grey to you? Is her face always this colour?'

At the height of summer, back in hospital in Hastings for a check-up, Marnie's condition again took a dramatic turn for the worse.

'I'll never forget it. There was a nurse there who took one look at her and said: 'I think this baby is going into heart failure.' And I thought: I knew it. And then I thought: oh God, I don't want to be right. And in an instant the room was full of people and there was talk of getting a helicopter.

'In the end we were blue-lit in an ambulance back up to London and the Evelina. That was two months ago. We haven't been home since.'

In another small, sombre room, for the first time there was talk of a heart transplant. Transplants in babies are rare — this year there have been just two in the UK — and the leading centre is Great Ormond Street Hospital, where Professor Michael Burch is head of cardiology.

It was clear Marnie met all the criteria to go on the list — chief among them that she won't survive until the age of two without a new heart.

'Doctors have made it very clear to us that she won't get even close to two,' says Amy. And yet the meeting with Professor Burch, when it came, was traumatic. 'He's an incredibly senior man,' says Nick, 'and he sat there and said to us: 'If there's one thing you take away, understand this: life expectancy of children who have heart transplants is 15 to 20 years.'

'A transplant isn't a cure,' says Amy quietly. 'It's a treatment plan.'

'And that's when our world came crashing down,' says Nick. 'Most people think getting a new heart will fix a child for ever, and it's what we thought, too. But it doesn't.'

Tragically, what it actually means is that they're quite likely to die as teenagers instead — although there is always the hope that having a second transplant, and perhaps one day even a third, could give Marnie a chance of a longer future.

'In fact, there are parents who decide not to place their very young children on the transplant list because they don't think a teenager could cope psychologically with knowing that they're going to die. So for their sake they don't do it.'

'It was at that point,' whispers Amy, 'that I just thought: how? How do you do this? I don't feel old enough or responsible enough to make this decision. How can you bring up a little girl and not let her dream of her future? Every time I start to think about it, I know I'm going to cry.'

'In the end it was a no-brainer,' says Nick. 'There are plenty of children who have normal, happy childhoods after transplant. Life could be wonderful — and we'd cope with whatever came after that. We have to give her the best chance we can.'

In the few hours spent away from Marnie, Nick and Amy have thrown themselves into the campaign to bring English law on organ donation into line with that in Wales, where a revolutionary change to the scheme was introduced last year.

Currently in England, potential donors aged over 18 have to opt into the nationwide organ donor scheme by registering their consent to donate after death — but in Wales, following landmark legislation, you have to expressly opt out or donation is automatic. For children, parental consent is still required in both countries.

According to NHS statistics, almost 49,000 people in the UK have waited for an organ transplant in the past ten years and more than 6,000, including 270 children, have died before receiving one. Wales's new scheme, also known as 'deemed consent', is expected to save hundreds of lives. Amy and Nick are urging everyone to sign a petition to force Parliament to debate a similar change in England.

HOW YOU CAN HELP Donate to the Braces’ fund by visiting: justgiving.com/crowdfunding/baby marniesnewheart Sign the petition on organ donation at: petition.parliament.uk/petitions/128473 Advertisement

'Wales had led the way and it seems obvious and sensible for England to follow,' says Amy.

But what they do most is wait and hope. When a heart becomes available for transplant, there is a four-hour time limit from the moment it is removed to surgery itself.

At all hours of the day and night a team at Great Ormond Street know exactly where Nick and Amy are — having mothballed business and home, they're living at Ronald McDonald House in Central London, run by a charity that provides free lodgings to parents of ill children.

By the time they get the call, an operating team will have been assembled and a theatre prepared.

And, of course, the call could come at any time.

'I don't sleep very well,' says Amy. 'I haven't for months. I have horrific dreams that wake me in the middle of the night. All we can do is believe this is going to happen for Marnie.