Description of the study population at the enrollment.

RDS: respiratory distress syndrome. DRG: diagnosis related groups at discharge. PROM: premature rupture of membrane. Numbers are mean(sd). P value from t-test. *n(%), p value from chi-square test. §complications were classified according to ICD-9 codes. §§pregnancy data were classified according to ICD-9 diagnosis codes. **n(%), p value from Fisher exact test. Description of the study population at the enrollment.