A mum has decided to give birth to her baby with a rare terminal illness despite doctors' warnings he likely won't survive more than a day.

Madysen Wilcox, 29 and her husband Darrin, 34, were shocked to hear third baby had alobar holoprosencephaly - a brain disorder - at their 18 week scan.

They were told the condition was fatal - and in the unlikely scenario he survived to birth, his life would be very short.

And after initially deciding to terminate the pregnancy they changed their mind.

Against the odds, Madysen will give birth to her son, Charlie, on Friday (4/10) via a scheduled induction.

With just 1 in 10,000 cases making it to birth, he is unlikely to survive much longer.

But the couple from Smithfield, Utah, are going ahead after praying for answers.

The stay-at-home mum from Smithfield, Utah, said: "It's been such a blessing to carry him."

"It's the hardest thing I've ever done but I have such peace of mind."

"It's been a really enlightening experience and something we never thought we'd have to go through and talk about things we never thought we'd have to talk about."

"What's crazy is I'm a planner through and through but it's so hard to face the birth and I haven't done any planning. "

"There's so much uncertainty. We don't know if he's going to make it through the birth, or live for a few hours, or a few days or a month even."

"This was going to be our last - it's blurred that for us. "

"We were done having kids and then this happened - we don't know if it will be all too traumatic to try again."

Madysen said she had problems with the baby shortly after finding out she was pregnant in February this year.

She said: "Two weeks after getting pregnant I had spotting and thought I was having a miscarriage."

"I went in for a scan and they found a subchorinic hematoma."

"It's a blood pocket that forms between the baby and uterus - they said it's fairly common and can be threatening to the pregnancy but mine was small."

"They put me on bed rest and it was a bit of a rollercoaster but it disappeared. "

"On the second scan, they could see that things weren't looking right at the start of the second trimester."

"They saw that the brain wasn't on track and they couldn't locate a nasal bone suspecting Down syndrome."

"This was a real shock. It was crazy."

"They did a genetic blood screening and came back negative."

At 18 weeks, Madysen and Darrin went to see a specialist and finally got a firm diagnosis - he had a condition in which the brain fails to split into two hemispheres.

Madysen said: "It's so rare, even the doctor didn't know a lot about it."

"Immediately we were told it was a terminal diagnosis. "

"I blacked out - it was like I was hovering above myself."

"I was thinking about what we'd tell our kids - I felt like I was going to vomit and I was sobbing uncontrollably."

"It was so hard to hear our daughter saying she can't wait for the baby to be born. "

"We told them he'd always be a part of our family and we'd always have an angel watching over us."

Faced with an "impossible decision", the couple initially chose to terminate but later changed their minds when they found out their insurance wouldn't cover the procedure at their chosen hospital - or perhaps within their state."

Madysen said: "We have very strong views on termination but when you're faced with this you're in a morally grey area - chances low, suffering high."

"We had a lot of roadblocks and we were getting answers to a prayer that we shouldn't do it and it wasn't right for our family."

"So we decided to carry him full-term and looking back and I'm so grateful that we did."

Madysen said her biggest fear after discovering her baby's diagnosis was people asking her about the pregnancy - and she wore big shirts to cover her bump.

She said: "If it was somebody that I was seeing a lot then I'd tell them the story and break the news by saying thank you, but we're carrying a baby with a terminal diagnosis."

"I'd often end up comforting them sometimes as it can be difficult to know what to say."

Her advice to others is to "stay in the present."

She added: "It's hard to give advice because it's so unique but if I could go back and tell myself something - stay in the present moment and don't think too far ahead."