New research confirms repeat doses of steroids given to pregnant women at risk of premature delivery are unlikely to harm the babies' health later in life.
Since the 1970s, antenatal steroids have been used to to mature the baby's organ systems.
Though the steroid treatment has already been shown to have important benefits, many doctors have been reluctant to use it more than once, due to fears the baby could be at more risk of developing health problems like diabetes and heart disease.
One of the researchers, Dr Chris McKinlay of Auckland University's Liggins Institute, said the study was the first to show the repeat steroid doses were safe, and doctors could be reassured the risk of health problems was low.
"With few treatments available for mothers at risk of preterm birth, this is an important finding," said Mr McKinlay.
"Forty percent of pregnant women at risk of having a premature birth and given a dose of antenatal steroids do not deliver immediately, and we wanted to know if giving repeat doses to these women improved outcomes for their babies without causing harm."
The study examined the cardiovascular and metabolic function of almost 260 premature New Zealand children of school age, who were exposed to repeat steroids or a placebo treatment before birth.
Researchers found the repeat dose of antenatal steroids did not increase risk factors for cardiovascular and metabolic diseases at early school age.
Mr McKinlay said the benefits of antenatal steroids for premature babies were many - they reduced the risk of lung disease, the need for a ventilator, infection, serious gut complications, and bleeding into the brain.
"All these factors combine to improve survival and prevent serious illness in preterm babies. Steroids such as betamethasone and dexamethasone cross the placenta into the babies' blood stream to act directly on fetal organs and cells," said Mr McKinlay.
"These synthetic steroids work by mimicking what normal happens in late pregnancy when the baby's natural steroid levels rise. This sends a signal to the fetal organs that it's time to start getting ready for life outside the womb."
He said steroids were still the most important treatment for premature babies, and have had a big impact on reducing complications in preterm babies.
Chris McKinlay said New Zealand researchers were the first to discover the steroid treatment, in the 1960s, and the first study on its use was carried out in Auckland by Professor Sir Graham Liggins and Professor Ross Howie, in 1972.
"Since those early days new questions have arisen and we now have to consider how treatments those babies received will affect them throughout life," he said.
"In the 1990s there was a shift in medical thinking about chronic disease when it was recognised that lifetime disease risk is linked to the conditions before and shortly after birth, especially poor growth."
He said this caused scientists to warn doctors about exposing premature babies to steroids, because they could play a role in changing how organs functioned in adulthood.