Multiple births have increased risks of birth defects compared to singletons.

Sex differences in birth defects exist between opposite-sex twins.

Introduction/Objective Preterm birth (PB) is the most important reason of neonatal mortality, and the second most common direct cause of death for children under the age of five years. The aim of this study was to analyze the clinical features and outcomes of preterm infants. Methods The clinical data of 307 preterm infants delivered in the Qingdao University hospital from January 1, 2012 to December 31, 2012 were retrospectively analyzed. Results The incidence of PB was 6.52%. There were 143 cases of preterm prelabour rupture of membranes (PPROM) (46.58%), 66 cases of spontaneous PB (21.5%), and 98 cases of therapeutic PB (31.92%). Deliveries with gestational weeks (GW) < 32 were mainly vaginal (60.72%), but deliveries with GW ≥ 32 exhibited higher C-section rate (60.99%) than the vaginal delivery rate (p < 0.05). The birth weight was 2,340.46 ± 606.26 g, and the Z-score at birth was-0.15 ± 1.08. The Z-score in the group with GW within 28 to 31 +6 weeks was less than that in the group with GW within 32 to 33 +6 and with GW ≥ 34 (р < 0.05). The average hospital stay of preterm infants was 15.17 ± 12.35 days, and the most common complication in these preterm infants was respiratory distress syndrome with 13.92%. Conclusion PB could cause a variety of serious complications in infants. The main causes of PB, such ас PPROM, should be actively prevented and treated; meanwhile, preterm infants should also be actively treated so as to improve their outcomes.