By Jacqueline Howard, CNN

(CNN) -- Courtney Stensrud and her husband call their fun-loving, spunky daughter a miracle.

The now 3-year-old girl was born at just 21 weeks and four days after conception. "She may be the most premature known survivor to date," according to a case report about her birth published in the journal Pediatrics last week.

In the United States, most pediatrics and obstetrics societies agree that 22 weeks of gestation is the lower threshold of viability, and many doctors recommend against assessing for viability or resuscitating babies born younger than 22 weeks due to a low chance of survival. Full-term babies are born at 39 through 40 weeks.

Before a medical emergency led to the early birth of her daughter in 2014, while still in the antepartum room at Methodist Children's Hospital in San Antonio, Texas, Stensrud said that she searched online for any other mothers who gave birth at 21 weeks.

"There were stories of 22-weekers, 23-weekers, but nothing about 21-weekers. So I knew that there was little to no survival or viability at 21 weeks," said the stay-at-home mom, now 35.

Just after Stensrud gave birth, Dr. Kaashif Ahmad, a MEDNAX-affiliated neonatologist at the hospital and lead author of the case report, counseled her about the baby's extremely low chances of survival and initially counseled against resuscitating the baby.

Stensrud listened as she held her 15-ounce girl in her arms, with the umbilical cord still attached, she said.

"Although I was listening to him, I just felt something inside of me say, 'Just have hope and have faith.' It didn't matter to me that she was 21 weeks and 4 days. I didn't care," Stensrud said.

"As he was talking to me, I just said, 'Will you try?' And he said he would, and three years later, we have our little miracle baby," Stensrud said.

"I don't tell her story a lot, but when I do, people are amazed," she said. "If there's another woman in antepartum that is searching Google, they can find this story and they can find a little bit of hope and a little bit of faith."

Stensrud requested that CNN not publish her daughter's name or current photos to respect her family's privacy.

Ahmad pointed out that Stensrud's daughter was one case, and more research needs to be done on preterm births lower than 22 weeks.

"We have to be very cautious about generalizing one good outcome to a larger population," Ahmad said.

"It is very possible that there have been many 21-week babies resuscitated in other places that did not have positive outcomes, and for that reason, we haven't heard about them," he said. "We reported this case because after this resuscitation she did well, but it may be possible that this is just an extraordinary case and that we shouldn't expect the same from other babies. We have to learn more before we can make any conclusions."

'She very slowly changed colors from blue to pink'

Around the world, an estimated 15 million babies are born too early -- before 37 weeks gestation -- every year, according to the World Health Organization.

Last year, preterm birth affected about one of every 10 infants born in the United States, according to the Centers for Disease Control and Prevention.

There has been an increase in the prevalence of preterm births in the United States, with an additional 8,000 babies being born prematurely last year due to a rise in the preterm birth rate between 2015 and 2016, according to a report released Wednesday from the nonprofit organization March of Dimes.

Pediatric and obstetric professional society guidelines are routinely updated to answer that challenging question of when resuscitating a preterm baby should be recommended or not.

The question also remains imperative for neonatologists who care for infants' medical problems. The answer to such a question remains tangled in concerns of ethics, health care costs and lifelong health outcomes for an infant.

Now, Ahmad hopes this latest case can help guide the pursuit for the right answer, he said.

In the new case report, Ahmad and his colleagues describe how they resuscitated Stensrud's daughter and how she needed prolonged care in the neonatal intensive care unit, known as the NICU. She wasn't discharged from the hospital until 126 days after being born.

Stensrud went into early labor due to a premature rupture of membranes and a common infection of the placental membrane called chorioamnionitis, according to the report.

When Ahmad and his colleagues entered Stensrud's labor and delivery room, they were not expecting to resuscitate the preterm baby, he said.

"But when the mother asked that we do everything for her daughter, despite having no reason to believe the baby would survive, I just made the decision to proceed with a vigorous resuscitation," Ahmad said.

"So we placed her under an overhead warmer, we listened, and we heard her heart rate, which we were not necessarily expecting," he said. "We immediately placed a breathing tube in her airway. We started giving her oxygen, and really pretty quickly, her heart rate began to rise. She very slowly changed colors from blue to pink, and she actually began to move and began to start breathing within a few minutes."

By 2 years old, even though she was smaller in size than her peers, Stensrud's daughter achieved scores that were average for a child around 20 months on Bayley III tests, according to the report. The tests, intended to measure child development up to age 3, assessed her cognitive, motor and language abilities.

"For this little girl, we say that her fine motor was age equivalent of 20 months," Ahmad said.

"That is what we would expect the average 20-month baby to do," he said. "She was at that time 24 months, but as we noted in the case, if you take into account how many weeks early she was, she was actually about 20 months, corrected."

She did not develop any auditory or visual impairments or cerebral palsy, according to the report, and she now attends preschool.

"If you didn't know that she was so preemie, you would think she's a normal 3-year-old," Stensrud said. "In her school, she is keeping up with all the other 3-year-olds. She loves playing with other kids. She loves everything I think a normal 3-year-old likes. She loves her baby dolls, she loves books, and she loves make-believe. She loves anything and everything her (older) brother is doing."

'The majority of infants born this early do not survive'

Though this baby girl's case appears to be "exceptional," there should be caution in assessing her outcome and even her true gestational age, said Dr. Noelle Younge, assistant professor of pediatrics at Duke University School of Medicine, who was not involved in the case report.

"Except in the case of assisted reproductive technology, there is always uncertainty about gestational age dating," Younge said.

"First-trimester ultrasounds are generally thought to be accurate within five to seven days, so it is possible the infant may have been 22 weeks of gestation at birth," she said. "As neonatal and obstetric care improve over time and a greater number of infants are actively treated at 22 weeks of gestation, there are likely to be more cases of infants who survive with favorable outcomes, but unfortunately, the majority of infants born this early do not survive."

Overall, there has been an increase in survival rates for preterm babies, according to a recent study led by Younge.

The percentage of infants born at 22 to 24 weeks' gestation who survived climbed from 30% around 2000 to 36% around 2011 across the United States, according to the study published in the New England Journal of Medicine in February.

The study involved data from the National Institute of Child Health and Human Development. The researchers assessed 4,274 births at 22 to 24 weeks of gestation in centers within the institute's Neonatal Research Network.

The researchers found not only that survival rates increased between 2000 and 2011 but that the percentage of infants who survived without neurodevelopmental problems increased from 16% to 20%.

"However, rates of poor outcomes remain high," Younge said. "Continued research into the causes, preventive measures and outcomes of periviable birth is critical. We need to continue to develop ways to improve outcomes for infants born extremely preterm."

One turning point in understanding the medical problems and survival of preterm infants came in 1963, Ahmad noted.

That year, President John F. Kennedy and first lady Jacqueline Kennedy "had a son named Patrick who was born at 34 weeks and had a breathing complication of prematurity that, for us, would be a very easy routine to take care of," Ahmad said.

"At that time just over 50 years ago, the technology and medicines available were not sufficient to save him, and he passed away within a matter of days," he said. "Since that time, we've had sustained improvements in care that have pushed the boundaries of how premature a baby can be born and not only survive but have a positive developmental outcome."

For now, Stensrud said that she hopes her baby girl's case inspires the world.

She didn't agree to tell her family's story for herself or her daughter, she said, but "for those other parents out there."

"From the moment she entered this world, she's just always wanted to live," Stensrud said of her daughter. "Now, she lives life."