When she was admitted to the hospital, she pulled her nurse down to her at bed level and whispered into her ear.
The nurse would later repeat the girl's words to comfort her family, as their worst fears were realised a day after Jenni's baby Chad Michael was born.
"She told the nurse, "I'm done, I did what I was supposed to. My baby is going to get here safe'," said Diana Phillips, Jenni's mother.
In photographs, the baby's ruddy cheeks and healthy weight offer a stark contrast to the frail girl who gave birth to him. Jenni, at five feet and four inches tall, weighed only 7st 10lbs at the full term of her pregnancy.
It was only 12 days after the birth that Jenni passed away in her bedroom.
Even so, her family and friends insist her legacy is not one centered in tragedy, but rather in sacrifice.
Recalling Jenni's infectious laugh and rebellious streak, her mother held the baby close, nuzzling his head, and said: "I want him to know everything about her, and what she did."
Jenny was 16 when she started getting migraines. She was taken to the family doctor, and an MRI scan found a small mass measuring about two centimeters wide on the right side of her brain.
She was sent to a hospital in Salt Lake City, some 150 miles south of her home in Pocatello, and another scan there showed the mass was bigger than previously thought.
Jenni had a biopsy on October 15, 2010, and five days later was diagnosed with stage three astrocytoma, a type of brain tumor.
With three tumors on her brain and three on her spine, Jenni was told her case was rare because the cancer had spread from her brain to another part of her body with no symptoms.
Her parents, who are divorced, remember they were brought into a room at the hospital and sat down at a long table as doctors discussed her chances of survival.
"Jenni just flat out asked them if she was going to die," said her father, Mike Lake, 43, a truck driver. "She didn't break down and cry or anything."
The answer wasn't good. With treatment, the teen was told she had a 30 per cent chance to make it two more years, Mr Lake said.
While he was heartbroken, Mr Lake marvelled at how strong she seemed in that moment.
But her mum recalled Jenni did have a weak moment that day. "When they told her that she might not be able to have kids, she got upset," said Mrs Phillips, 39.
Jenni started aggressive chemotherapy and radiation treatments, while also posting videos on a YouTube site titled Jenni's Journey, where she hoped to share her story with updates every other day.
She managed to upload only three videos, though, as her treatments left her tired and weak.
By March of this year, the tumors had started to shrink, the family said.
Jenni had started dating Nathan Wittman a couple of weeks before she received her diagnosis.
Their adolescent relationship withstood the very adult test posed by cancer, the treatments that left her barely able to walk from her living room to her bedroom but the young couple dreamed of someday opening a restaurant or a gallery.
And then Jenni discovered she was 10 weeks pregnant. Her journey was no longer her own.
From the start of treatment, she was told that she might never have children, her mother said, that the radiation and chemotherapy could essentially make her sterile.
"We were told that she couldn't get pregnant, so we didn't worry about it," said Nathan, 19, who now has custody of their son Chad.
Jenni, the third of her parents' eight children, had always wanted to be a mum.
"The oncologist told us that if she's pregnant, she can't continue the treatments,' Diana said. "So she would either have to terminate the pregnancy and continue the treatments, or stop the treatments, knowing that it could continue to grow again."
Her oncologist, Dr David Ririe, would not discuss Jenni's care, citing privacy laws, but said, generally, in cases in which a cancer patient is pregnant, oncologists will consider both the risks and benefits of continuing with treatment, such as chemotherapy.
"There are times during pregnancy in some situations, breast cancer being the classic example, where the benefits of chemotherapy may outweigh the risk to mother and baby," Dr Ririe said. "There are other times where the risk outweighs the benefits."
There was no discussion about which path Jenni would choose. Her parents didn't think of it as a clear life or death decision, and Jenni may not have, either.
They believed that since the tumors had already started to shrink earlier, she had a strong chance of carrying the baby and then returning to treatment after he was born.
"I guess we were just hoping that after she had the baby, she could go back on the chemotherapy and get better," her mother said.
Jenni and Nathan named the baby Chad Michael, after their dads. Nathan has legal custody of the child, who is primarily cared for by Nathan's mother, Alexia Wittman, 51.
Jenni didn't show regret for her decision, not in the final weeks of her pregnancy as she grew weaker, and not when she started to lose her vision as the cancer took its course.
Jenni's last words were about her son. As she felt for the baby to snuggle beside her, she said: "I can kind of see him."