The most premature baby ever born is defying doctors' expectations with her incredibly strong health three years on, according to a new medical report on the unprecedented case.

According to the Daily Mail, the little girl was born at 21 weeks and four days in San Antonio, Texas, in 2014.

Her mother Courtney Stensrud, now 35, had been hospitalised with a common placental infection and ruptured membranes at just three months pregnant, and was told she had to deliver immediately to avoid losing the baby.

Lying in the maternity ward, Stensrud remembers frantically searching on her phone for examples of any other babies born at 21 weeks, to no avail.

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But now, against all odds, the little girl who weighed just 0.425 kgs at birth is thriving, attending pre-school, has no medical conditions, and is scoring just as well on developmental tests as any other child her age.

The report on this case comes just days after new data from The March of Dimes revealed that the premature birth rate in the US has increased for the second year in a row following nearly a decade of decline.

"If you didn't know that she was so preemie, you would think she's a normal three-year-old," Stensrud told CNN.

The little girl was born at 21 weeks and four days in San Antonio, Texas, in 2014. Photo / Courtney Stensrud
The little girl was born at 21 weeks and four days in San Antonio, Texas, in 2014. Photo / Courtney Stensrud

"In her school, she is keeping up with all the other three-year-olds. She loves playing with other kids. She loves everything I think a normal three-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."

Analyzing the case in Pediatrics today, Stensrud's doctors said this is just one example and not all premature births have such positive outcomes - but this story can be used as a benchmark to learn from to avoid future preterm birth fatalies.

"We have to be very cautious about generalising one good outcome to a larger population," Stensrud's neonatologist at Methodist Children's Hospital Dr Kaashif Ahmad told CNN.

"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."

The new report explains that Stensrud's hospitalisation was triggered by a common infection called chorioamnionitis, or intra-amniotic infection, which affects the placental membrane.

The infection is an inflammation of the fetal membranes, typically caused by a bacterial infection stemming from the vagina.

If contracted later in the pregnancy, it can affect the birth by prolonging labor.
But in Stensrud's case, it meant premature labor.

After the little girl was born, small enough to loop her father's wedding ring around her arm, she was blue.

Dr Ahmad admits he did not have high expectations that the baby would survive, and had never resuscitated a baby so premature.

They placed her under a heating device, put in a breathing tube and monitored her heart rate. Gradually her body turned pink and she began to breathe.

While Stensrud has asked not to publish current pictures of their family to protect their privacy, she said she was keen to share their story to give other mothers hope.

"I don't tell her story a lot, but when I do, people are amazed,' she told CNN. "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."

A pregnancy normally lasts 40 weeks. Preterm birth is classified as any time before the 37th week.