A Texas Army veteran whose baby has a life-threatening brain tumor is battling her insurance company over its last-minute refusal to pay for an operation at the acclaimed hospital that's been treating the child.

Savannah Snodgrass was just four months old in March, when doctors discovered the tumor in her left temporal lobe and sent her to Texas Children's Hospital. Deeming Savannah too young and fragile for brain surgery, doctors monitored her closely only to find the tumor growing. This week, the team of specialists there ordered emergency surgery to be performed on Tuesday, but the operation was scuttled when Superior HealthPlan, a Texas-based HMO, wrote a letter to Tessa Snodgrass denying coverage at the hospital.

"I was shocked," Snodgrass, of Georgetown, Texas, told FoxNews.com. "You don’t expect something like this to ever happen. I was absolutely stunned because they had been sending us to Texas Children's all this time."

Snodgrass, a 38-year-old mother of four and retired U.S. Army major, said it all started when Savannah began having seizures in March. She was first taken to her pediatrician who ordered she be admitted to a hospital in Temple, Texas, after she had 15 seizures in a 24-hour period, her mother said.

Doctors there referred the Snodgrasses to Texas Children's Hospital where doctors mapped out the plan to monitor the tumor closely while allowing for Savannah to grow in size before operating.

An MRI conducted two weeks ago confirmed the worst: the tumor had grown substantially, leading doctors to suspect malignancy and order emergency surgery to remove and biopsy the mass, the family said.

Snodgrass claims Superior HealthPlan had been covering Savannah's care all along -- including pre-approval for another MRI to be conducted on Monday.

But in a "stunning" turn of events, Snodgrass claims the company refused to pay for Savannah's brain surgery scheduled for Tuesday, claiming in a June 27 letter that Texas Children's Hospital is an out-of-network provider and referring her instead to a surgical facility in Austin, about an hour south of Georgetown.

Snodgrass believes her daughter's best chance lies with the doctors at Texas Children's, who have been tending to her for months.

"It's rare for a tumor to be found in a child this young," she said. "It so clearly called for a neurosurgeon specialized in operating on babies -- not just any surgeon."

"The average national survival rate for infants for this surgery is 47 percent. At Texas Children's Hospital, survival rate is close to 90 percent," Snodgrass said. "Would you accept less than a 50-50 survival rate when you can have almost 100 percent?"

"If we had more notice, we could have changed plans months ago," she said.

In an e-mail to FoxNews.com, Superior HealthPlan said it is "confident" the family will find capable in-network providers.

"Paramount to Superior HealthPlan is helping our members achieve positive health outcomes," the insurance company said a statement Wednesday. "We are sensitive to the complex and timely concerns of this family and are actively working with them to ensure their daughter receives the highest quality of care.

"While we cannot disclose the details of this case due to privacy rules, we are confident that our in-network providers – including Dell Children’s Hospital – have the expertise and resources to give each member the individual care they need," the company said. "We too want the best for this child and will continue to do everything in our power to assist the family in this endeavor. “

Snodgrass, who served two deployments in Iraq, said she's been insured by the company for almost two years. She claimed that after a lengthy delay with the U.S. Department of Veterans Affairs, she finally received a medical retirement plan from the military for injuries she sustained in combat but said that plan, which would cover her family, will not take effect until August.