An Australian woman has given birth to four healthy identical babies following an “extremely” risky pregnancy with “one-in-15 million” odds.

The quadruplets – named Emily, Harriet, Catherine, and Alexa – were delivered prematurely by caesarean section at the Royal Brisbane and Women’s Hospital on 14 July. They were born at 28 weeks and four days – more than two months shy of the full term.

Dr Alexa Bendall, who has cared for mother Jenitar Na’amoana, 34, since the start of her surprise pregnancy, said: “We always said that if we could get her to 28 weeks, we would be doing well.”

“So the fact that we’ve got four beautiful, healthy babies born at 28 weeks and four days is incredible and we’re very happy to be a part of it,” she added. Dr Bendall said it was a “beautiful tribute” to have one of the girls named after her.

The infants remain in the neonatal intensive care unit, where doctors say they are doing “extremely well”. They are expected to remain until they grow to full term, Dr Bendall said.

The rare pregnancy involved a single fertilised egg splitting into four embryos, the hospital noted, making the naturally conceived babies monozygotic, or identical, quadruplets.

Dr Bendall said she had never encountered such a case before.

“This is unheard of. We don’t think it has ever happened in Australia,” the doctor said, according to the news agency AAP.

“There may have been a couple of cases elsewhere in the world. And when I say a couple, literally two.”

Dr Bendall described the pregnancy as extremely high-risk because the four babies shared a single placenta, a situation that could cause complications for both the mother and the children.

“There’s a high risk of mortality and morbidity for these babies and for mum,” she said, according to 7 News.

Ms Na’amoana was admitted to hospital at 25 weeks so doctors could closely monitor her condition. The medical team had hoped to safely reach 28 weeks of pregnancy.

“When they went along and found out they had four babies instead of one they were quite shocked,” Dr Bendall said.

“We have never seen a spontaneously conceived pregnancy of this type with quads and she’s somehow dodged every complication and risk for both herself and her babies to be here today, with four gorgeous newborn girls.”

Hospital staff said the babies were expected to remain in intensive care until they were closer to full term but had made an encouraging start.

Ms Na’amoana has started a GoFundMe campaign to help cover the costs of raising the four babies at once, including medical expenses, travel, everyday living costs, and a larger vehicle capable of seating the expanded family.

“Our four precious miracles have arrived safely,” she wrote on the fundraising page. “While our hearts are overflowing with gratitude, the reality of caring for four newborns at once has brought challenges we never could have imagined.”

“We urgently need a larger van with 10 or 12 seats, essential baby equipment, and help with ongoing medical, travel, and everyday living expenses.”

Ms Na’amoana and her husband already had four children before the surprise pregnancy. The older children are between one and 10 years old.