Four-month-old Kallie Bender's life has been full of firsts.

There was her first bottle. Her first bath in a "big girl" tub. The first time she could fit into newborn clothes.

Those might seem like typical milestones for any newborn, but for this baby, born at just 25 weeks gestation and weighing less than 1 pound, it's nothing short of a miracle.

And Monday is the sweetest first of all: baby Kallie is headed home for the first time.

She was born on May 24, 2019, 15 weeks early and one of the smallest babies ever born at Dignity Health St. Joseph's Hospital & Medical Center, the Phoenix, Arizona hospital told "Good Morning America."

Her mom, Ebonie Bender, was admitted to the hospital when she found out Kallie was measuring small during an ultrasound appointment. There was also a lack of fluid around the baby, caused by a condition called absent end diastolic flow, the hospital said.

In addition to being a micro-preemie, Kallie also survived a successful heart procedure for a patent ductus arteriosus (PDA), a common heart defect among babies born as early as she.

"She's a feisty girl," Bender told "GMA."

Kallie's mom was "scared" at the time of her birth," she said. "It was so much fear of the unknown."

For several weeks after her birth, Kallie relied on a machine to help her breathe. She was intubated and needed to learn to eat.

Dr. Vinit Manuel, medical director of the nursery intensive care unit (NICU) at Dignity Health St. Joseph’s Hospital and Medical Center, told "GMA" that the hospital's NIDCAP -- Newborn Individualized Developmental Care and Assessment Plan -- is a teaching program for other NICUs in Arizona.

It "looks at each baby as an individual," he said. From the delivery room to pharmacists to the nurses to the occupational and physical therapists, he said, the team works together to meet each baby's needs.

However, Manuel said, "No technology can replace the womb."

For that reason, he said, "the involvement of the family in the care of these babies is crucial."

The Benders, who have have three sons in addition to baby Kallie, were "very involved from day one. They were by her bedside, reading to her. It's not possible to quantify, but research shows it's very important for the baby's development."

For the nurses dedicated to her care, seeing Kallie go home is a joy.

"We're thrilled that after nearly five months Kallie is going home with he family," said Becky Cole, one of Kallie's primary nurses. "We've loved being able to watch her grow and are excited for her to celebrate many milestones in the future with her parents and brothers."

Today, Kallie weighs over 7 pounds and is thriving.

"It's bittersweet to leave," her mom told "GMA." "But she's an amazing little fighter and I can't wait to see the personality she brings to our family."