A Monmouthshire couple plan to use the umbilical cord of the baby they are expecting for stem cell treatment for the unborn child's 20-year-old brother.
Joanna Stanley and Michael Emms, her partner's son
Joanna Stanley, 27, is pregnant by partner Julian Emms, 41, whose other son, Michael, has been diagnosed with motor neurone disease.
It is hoped the baby due in September, a boy, already named Rhys, will be a match for the younger Mr Emms, 20.
The treatment will be performed in America as it is illegal in the UK.
Experts believe the cells could reverse the muscle wastage Mr Emms has suffered since the illness was diagnosed.
The plan is to freeze stem cells from baby Rhys's umbilical cord and fly them to a clinic at the Institute of Cellular Medicine, in Boston, where they will be implanted into Mr Emms' spinal cord.
Mr Emms, of Caerwent near Newport, has been told he will not see his 21st birthday without the treatment.
Ms Stanley said she was six-weeks pregnant when she read on the internet about stem cell treatment for motor neurone disease.
She said: "Mike's best chance is to have cells from a donor who is a close genetic match. The best he is going to get is from his own brother even though he is not born yet.
Julian Emms and Joanna Stanley admit it's a 'long shot'
"To get a match otherwise could take three or four years and Michael doesn't have years, he only has months.
"It's a race against time but Rhys will have a DNA link. We know it's a long shot and we are all hoping against hope."
Julian Emms said: "This disease is meant to affect old people. Not my young and healthy son."
Michael Emms is losing the mobility in his limbs, especially in his hands and suffers from extreme fatigue.
The Motor Neurone Disease Association (MNDA) say most sufferers die within two to five years of diagnosis - with half dying within 14 months.
Julian said: "When doctors first diagnosed Mike, none of us knew what it was.
"Then when we did some research, we couldn't get our heads round it. This disease is meant to affect old people. Not my young and healthy son.
"It would be just amazing if little Rhys saved Michael. Rhys will be our miracle baby."
Motor neurone disease affects about 5,000 people in the UK. It is most common among people aged 50 to 70, and tends to affect men slightly more than women.
It is a progressive disorder caused by the breakdown of the nerve cells which control muscle activity.
People with the condition suffer muscle-wasting, loss of mobility, and difficulties with speech, swallowing and breathing.
Stem cell research has been condemned by pro-life campaigners and church groups as it often involves harvesting cells from human embryos.
Stem cell therapy is being used to treat motor neurone disease patients in the US and Canada.
But it has yet to be approved by UK health authorities because doctors are split on its effectiveness.
A spokeswoman for the MNDA said: "We are not for or against the treatment but it is at very early stages and not proven that it can have any effect on motor neurone disease."