A simple test which takes five minutes and costs no more than $4 is saving several newborn babies' lives a year in New Zealand.

But there's a catch, researchers have found. Many babies are missing out because midwives in their area are too stretched or because the family is not registered with a maternity carer.

For the last two years, babies born in at Auckland, Counties Manukau or Lakes district health boards have been getting an extra check from their midwife, called pulse oximetry.

A small, band-aid like device is strapped around their foot, which indicates the oxygen level in their blood. If the oxygen level is low (normal levels are 95 per cent or more) it could indicate critical heart defects and in these cases urgent surgery is carried out.

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"The test is extremely useful - that is our first finding," said Liggins Institute neonatologist Dr Elze Cloete, who has been leading a study on the screening programme.

Out of 16,664 babies who were screened, 31 had defects. Four of them had critical heart problems, and the early intervention likely saved their lives.

Tu'amelie Faingata'a was one of those babies. Just two weeks after the pulse oximetry trials started, he was found to have low oxygen after his birth at Auckland Hospital. An X-ray and scan showed he had transposition of the great arteries, which meant the main heart valves were around the wrong way. It would kill him within days if untreated.

"At first I was really scared," said his mother Silivia, from Mt Wellington. "But if it wasn't for the test I don't know whether he would have lived or died."

Tu'amelie had surgery at 6-days-old to repair his heart, and later had a second operation. Now 3-years-old, he is thriving.

"He is jumping around and very active," Silivia said. "He is just like a normal boy, just like his older brothers."

 Newborn Tu'amelie Faingata'a at Starship Hospital. Photo / Doug Sherring
Newborn Tu'amelie Faingata'a at Starship Hospital. Photo / Doug Sherring

The case for screening babies at all DHBs is now building. The Ministry of Health said it was developing national guidelines for pulse oximetry which were expected to be completed late next year.

Cloete said the study highlighted some challenges for a national-level screening programme. Some newborn babies were missing out on screening in the current trial because of unequal access across DHBs and populations. The main causes were midwife shortages, overworked midwives, and failure to register with a maternity carer.

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Counties Manukau was included in the trial, but Middlemore Hospital midwives were simply too stretched to add the test to their list of demands.

NZ College of Midwives adviser Lesley Dixon, who took part in the research, said midwives were highly supportive of screening. But midwives would have to be properly resourced for it to work at a national level, she said.

"The test is extremely useful": Liggins Institute neonatologist Elza Cloete says pulse oximetry picked up four critical heart defects in babies over two years. Photo / Supplied

Midwives had roughly 20 tasks after the birth of the baby, and their assessments took between two and three hours. That followed labour of anywhere between two hours and 18 hours, Dixon said.

At the moment, heart defects are detected in two main ways - through an ultrasound at around 20 weeks of pregnancy, or through a physical examination immediately after birth.

Both methods have their limits. Not everyone gets an ultrasound during their pregnancy. And physical symptoms may not show up for 24 hours after birth, when the parents have often gone home.