Babies born between 34 weeks and 37 weeks gestation are much more likely to have respiratory illness compared to infants born at full term, and their risk of respiratory illness decreases with each additional week of gestation until 38 weeks, researchers report.

The study, published in the July 28 issue of JAMA, was conducted by University of Illinois at Chicago researcher Dr. Judith Hibbard and colleagues from the Consortium on Safe Labor.

For neonates born at 34 weeks, the odds of respiratory distress syndrome (RDS) were increased 40-fold.

"Even at 37 weeks, babies were three times more likely to have respiratory distress syndrome compared to babies born at 39- or 40 weeks," said Hibbard, UIC professor of obstetrics and gynecology and lead author of the study.

Contrary to earlier research, the researchers did not find a significant increase in respiratory illnesses in babies born at 38 weeks, compared with babies born at 39 and 40 weeks gestation, after controlling for multiple factors. Data were collected from electronic medical records on 233,844 deliveries at 19 hospitals across the U.S. between 2002 and 2008.

Hibbard said concern has grown in recent years about the problems associated with late preterm birth and the increasing number of babies delivered early. Respiratory illnesses such as RDS, transient tachypnea, pneumonia and respiratory failure can lead to other problems such as longer hospitalization, the need for a ventilator or antibiotics, and issues with feeding and failure to gain weight.

Medical experts suspect that the increase in late preterm births may be due in part to "convenience" c-sections and induced deliveries that are done, in some cases, "without good medical reason," said Hibbard.

The researchers looked at all newborns 34 weeks or greater with respiratory problems admitted to neonatal intensive care units. Late preterm births were compared with full-term births for resuscitation, respiratory support and respiratory diagnosis.

Using a statistical model, the researchers examined infant respiratory illnesses at each gestational week, controlling for factors that influence respiratory outcomes including maternal medical conditions, length of labor and mode of delivery, and birth weight.

The study found that late preterm births accounted for 9 percent of all deliveries. Thirty-seven percent of late preterm infants were admitted to the neonatal intensive care unit, compared with 7 percent of term infants.

Overall, respiratory illness occurred in 9 percent of late preterm births in the study.

RDS was the most common respiratory illness, occurring in 11 percent of 34-week deliveries. Only 0.3 percent of 40-week deliveries had RDS.

Transient tachypnea, also called "wet lungs," was the second most common respiratory illness, occurring in 6.4 percent of 34-week deliveries, and decreasing to 0.3 percent at 39 weeks.

"The OB community needs to assess indications for induction of labor," said Hibbard, who hopes that this study will help clinicians to counsel their patients about the importance of not requesting medically unnecessary inductions.

Hibbard suggests that further prospective research needs to be conducted to determine if it may be useful to use steroids to promote fetal lung maturity beyond the current standard of 34 weeks or if it may be useful to use medications to stop premature labor in women beyond 34 weeks to maintain pregnancy longer.