Ground-breaking New Zealand research will have a potentially life-saving effect on the treatment of premature babies around the world.
Many doctors have been reluctant to give repeat doses of antenatal steroids to women at risk of delivering before 32 weeks.
The steroids help to mature a baby's organ systems, especially the lungs, gut and cardiovascular and immune systems.
However, some scientists have cautioned against repeated steroid doses in the belief they may cause health problems in later life.
Now, University of Auckland researchers have proven that repeated doses are safe - clearing the way for more babies to receive the treatment, which can save lives.
"With few treatments available for mothers at risk of pre-term birth, this is an important finding," said Dr Chris McKinlay, a neonatologist and researcher with the university's Liggins Institute.
"Steroids remain the most important treatment for premature babies and have a big impact on reducing complications in pre-term births."
Premature births account for up to 8 per cent of all deliveries in New Zealand. About 800 births a year are before 32 weeks - the very pre-term group that would likely be targeted with repeat steroids.
The study's findings were published this month in the American journal Paediatrics.
It is the first to show that repeat steroid doses do not adversely impact the later cardiovascular and metabolic health of babies.
The World Health Organisation has estimated that if every eligible mother got steroids when there is a risk of a premature birth, it would save around 400,000 lives a year.
Dr McKinlay said repeat doses also had a significant effect.
"The thing babies struggle with the most when they are born is that their lungs are too immature and they struggle to breathe. We're able to reduce severe pre-term lung disease by 40 per cent [with] repeat doses. But we had to know that benefit wasn't traded off by problems later on."
The study followed up all the New Zealand children aged 6 to 8 in a previous study where they were exposed to repeat steroids or a control placebo before birth.
Dr McKinlay and a research team tracked them down by phone, sleuthing on Facebook and "a fair bit of door-knocking".
All of the children bar six were found, and 81 per cent agreed to take part in up to seven hours of testing.
The field work gathering data from the 258 children took 2 years.
The use of steroids for babies at risk of premature birth was pioneered by a study done in Auckland by Professor Sir Graham (Mont) Liggins and Associate Professor Ross Howie and published in 1972.
Family saw chance for good results
The Vince family were happy to help when asked to participate in research on the long-term health effects of steroids on premature babies.
Jack, now 11, spent his first 97 days at the National Women's Hospital in Greenlane after being born early at 28 weeks, five days.
His mother Marion was given two steroid shots before his birth, and agreed to take part in a study which saw researchers give women repeat steroids or a control placebo.
Jack, who is in the enrichment class at Glenfield Intermediate and wants to be a neonatologist, was on oxygen for his first five months.
"It was a rollercoaster journey, two steps forward and then one step back," Mrs Vince recalled.
A couple of years ago the family learned the dose was a placebo. Participating in both studies was done in the knowledge it would help others, said Jack's father, Mark.
"To know that it has made a difference for people in the future and families who are going to go through the same type of process, that's great."
Drug helps to mature organs
What has the study shown?
The New Zealand study is the first to show that repeat antenatal steroid doses given to mothers at risk of delivering before 32 weeks does not adversely affect the later cardiovascular and metabolic health of premature babies.
Why are steroids given to mums at risk of premature delivery?
The steroids help mature a baby's organ systems, especially the lungs, gut and cardiovascular and immune systems. The synthetic drugs mimic what normally happens in late pregnancy when the baby's natural steroid levels rise.
Why was there reluctance to prescribe repeat steroid doses?
A shift in medical thinking in the 1990s recognised that lifetime chronic disease risk is linked to the conditions before and shortly after birth, especially poor growth.