Babies born between 34 and 37 weeks into pregnancy are much more likely to suffer from respiratory illness than those born at full term, a study has found.
Scientists at the University of Illinois, Chicago (UIC) also discovered that premature babies' risk of breathing difficulties decreases for every additional week they remain in the womb until 38 weeks.
Their study shows that the chances of respiratory distress syndrome (RDS) are increased 40-fold for babies born at 34 weeks.
Lead author Dr Judith Hibbard, professor of obstetrics and gynaecology at UIC, revealed: 'Even at 37 weeks, babies were three times more likely to have respiratory distress syndrome compared to babies born at 39 or 40 weeks.'
The findings are published in the Journal of the American Medical Association and were based on data on 233,844 deliveries at 19 US hospitals between 2002 and 2008.
They coincide with the news that a set of quadruplets have all been allowed home for the first time after being born by emergency caesarean at 28 weeks in January.
Three of the baby girls have chronic lung disease and require daily sessions on an oxygen generator.