Teen mum paused her cancer treatment to give birth. Now, she's been told she is dying.

A young New Zealand mother who chose to forgo cancer treatment to save the life of her unborn son has been told there is little chance she will ever be cured.

Rhianna Truman is just 17, and she has no clear idea of how much time she has left with her four-month old son, Daeshaun.

Truman was diagnosed with adamantinoma​, an extremely rare bone cancer, in 2012. She had an operation to remove the bone in her leg, and a metal rod was put in its place.

In 2015 she was told the cancer had spread to her lungs.

When she fell pregnant with her son, she chose to put her cancer treatment on hold to keep him safe, despite knowing she was putting her own health at risk.

"When I found out I had cancer I was scared," Truman told stuff.co.nz.

"But when I found out I was pregnant, I was like, 'This is my reason, this is my chance to make something good out of my life, and be a proud mother, and for him to eventually be proud of me'."

Last week, she was told that her days with her newborn son "were numbered".

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"There's no cure for me," she said.

"My doctor just said to live my life, and make my bucket list and do it now, while I have the chance to spend time with my son."

Rhianna has since set up a charity page to help her make her final few weeks with her son memorable.

"I have recently been told that finding a cure for my Cancer is like 'finding a needle in a haystack' and because of this I need your help not just for me but to leave lasting memories for my baby boy," she wrote on the page.

"I have decided to set up a page to help me make memories with my son and live our lives with as much fun and happiness while I'm well enough to enjoy it."

Despite knowing there is no cure for her disease, Rhianna said she is not defined by her cancer.

"You are still the same person, you just have to live each day as it comes. My son is my all," she said.

"Being a mum is indescribable. He's more than my world, he's everything I live for."