Celebrity Trainer Gunnar Peterson Shares ‘Nightmare’ That His Daughter, 4, Has Leukemia: 'Into the Storm'

Monroe Vivian is undergoing treatment for AML leukemia, but her dad says “our Warrior Princess will crush this”

The four-year-old daughter of celebrity trainer Gunnar Peterson has leukemia, and is currently undergoing treatment.

Peterson, who has worked with stars like Khloé Kardashian and Jennifer Lopez, shared the news in an Instagram post Wednesday that his daughter, Monroe Vivian, had been diagnosed with acute myelogenous leukemia (AML Leukemia), which the Mayo Clinic says is cancer of the blood and bone marrow.

“This is going to be longer, and cathartically different from my usual fare. Forewarned is forewarned. Life changing turn,” Peterson began his post, which featured photos of his family with Monroe in the hospital.

“Our perfect 4 year old daughter, Monroe Vivian Peterson (MVP, see that?!) had a nagging cough, on and off fever, and an odd pallor to her for a few days last week. Typical kid stuff.”

When his wife Jess noticed bruises on Monroe’s legs, Peterson said she took their daughter to the hospital, as the bruising “was odd since she hadn’t been very active because of the other symptoms.”

Four days later, he wrote, Monroe’s white blood cell, platelet, hemoglobin, and hematocrit (percentage of red blood cells) counts dropped — requiring Monroe to undergo a series of treatments.

As the Mayo Clinic explains, “the word 'acute' in acute myelogenous leukemia denotes the disease's rapid progression.”

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That was the case for the Petersons; the trainer wrote Monroe needed prompt treatment — and tests to diagnose what was going on. They included “immediate transfusion(s). Has to have bone marrow biopsy. Flow cytometry. Spinal tap(s). Chemotherapy. And finally, Make-A-Wish candidate.”

“She has AML leukemia,” he wrote. “How the f*#! did we get here?!?!”

There are some known risk factors for AML leukemia, according to the American Cancer Society, but none of them — like smoking, getting older, or being male — apply to Monroe, who just turned 4 in December.

“More to come as we navigate the nightmare… Our Warrior Princess will crush this! She has even said it herself,” said Peterson, a member of the PEOPLE health squad.

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“Please hold our daughter, and our family — immediate and extended — in your hearts, thoughts, and prayers. As we are all aware; ‘you never know what someone is going through.’”

He ended his post by saying he’ll return to his usual “levity and fitness” posts, but said he would share “more on this as it unfolds. Into the storm.”

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