Infants were defined as extremely preterm if they were born after 22-28 weeks of gestation.
The study, published in JAMA, also reports an increase in survival without major complications among infants born after 25-28 weeks of gestation.
Dr. Barbara Stoll, of the Emory University School of Medicine in Atlanta, GA, and colleagues reviewed data for extremely preterm infants born at Neonatal Research Network hospitals between 1993 and 2012.
A total of 34,636 infants were defined as extremely preterm, being born after 22-28 weeks' gestation, with birth weights between 14.1 ounces and 3.3 lbs.
"The study provides a global overview and level of detail not presented in earlier studies," state the authors. "Findings demonstrate that progress is being made and outcomes of the most immature infants are improving. These findings are valuable in counseling families and developing novel interventions."
Markers of progress include the increasing prevalence of measures taken to improve outcomes at birth. The researchers found that the use of antenatal corticosteroids increased considerably, rising from use in 24% of cases in 1993 to 87% of cases in 2012.
A similar increase was observed for Cesarean delivery - also known as a C-section - rising from 44% in 1993 to 64% in 2012. Cesarean delivery can be recommended for a variety of reasons, such as if the baby is thought not to be getting enough oxygen.
The adoption of strategies to reduce the risk of lung injury is also an indication of improved outcomes. The prevalence of delivery room intubation fell from 80% in 1993 to 65% in 2012, and rates of continuous positive airway pressure without ventilation rose from 7% in 2002 to 11% in 2012. These findings demonstrate that less aggressive ventilation strategies are being pursued.
Rates of bronchopulmonary dysplasia, a lung disease that develops after mechanical ventilation or oxygen inhalation therapy, increased for infants born after 26-27 weeks' gestation between 2009 and 2012. Despite this, the rates of other complications, such as late-onset sepsis, declined.
'Progress is being made'
Between 2009 and 2012, survival increased from 27% to 33% for infants born at 23 weeks' gestation, and from 63% to 65% for infants born at 24 weeks. Small increases were also observed for infants born at 25 and 27 weeks' gestation, and no changes were reported for infants born after 22, 26 and 28 weeks.
For infants born after 25-28 weeks' gestation, survival without major complications increased by around 2% per year. No change was observed for those born at 22-24 weeks. The authors suggest that this may be the most important new finding.
"Although overall survival increased for infants aged 23 and 24 weeks, few infants younger than 25 weeks' gestational age survived without major neonatal morbidity, underscoring the continued need for interventions to improve outcomes for the most immature infants," they write.
In an accompanying editorial, Dr. Roger F. Soll, of the University of Vermont College of Medicine in Burlington, writes that more work needs to be done to improve these outcomes:
"It is clear that there are still a substantial number of extremely preterm infants who either die or survive after experiencing one or more major neonatal morbidities known to be associated with both short- and long-term adverse consequences. Although the neonatal-perinatal medicine community can be proud of the progress made, an additional commitment must be made to further improvements in the decades to come."
Previously, Medical News Today reported on a study suggesting that vaginal microbes could help clinicians predict preterm births. Women with a lower level of the vaginal bacteria Lactobacillus were found to have an increased risk of delivering their baby preterm.