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Objective To investigate the changes in preterm birth rate,its gestational age distribution,and possible contributors in Peking Union Medical College Hospital (PUMCH) over the last 25-year period. Methods The clinical data of premature deliveries,both singleton and twins,in PUMCH from January 1,1990 to December 31,2014 were retrospectively analyzed. We counted the number of premature fetuses and assessed the changes of preterm birth rate and its gestational age distribution (including extremely preterm birth,early preterm birth,and late preterm birth) over time. The etiologies (including spontaneous and iatrogenic) of preterm birth were also surveyed. Results The overall preterm birth rate was 7.8% in PUMCH,showing a slightly up-trend in both singletons and twins. Twin prematurity accounted for 23.8% of total preterm births,increased from 15.1% to 28.5%. Preterm births subgrouped by gestational age included 26 cases (0.7%) of extreme prematurity (<28 weeks),1199 cases (33.9%) of early preterm birth (28- 33 +6 weeks),and 2310 cases (65.3%) of late preterm birth (34- 36 +6 weeks). The gestational age distribution in singletons and twins showed no significant difference(z=0.844,P=0.398). Changes in the proportion of preterm birth before 28 weeks was little,gradually increased in the 28- 33 +6 weeks group (from 23.8% to 36.1%) and gradually decreased in the 34- 36 +6 weeks group (from 75.5% to 63.3%). Trends of gestational age distribution of singleton and twins were similar to that of the total. Spontaneous preterm labor,preterm premature rupture of membrane,and medically indicated (iatrogenic) preterm birth accounted for 20.2%,38.9%,and 40.9% respectively. There was no difference in singletons and twins(χ 2 =1.071,P=0.301).The proportion of iatrogenic preterm was increased. Common reasons for iatrogenic preterm birth included gestational hypertension,fetal indications (including fetal distress,fetal growth restriction),placenta previa,and pregnancy complicated by heart disease. Conclusions The overall preterm birth rate shows an upward trend in the general hospital as a result of more multifetal gestations and more medically indicated preterm births. Reducing multifetal gestations and effective control of pregnancy complications should be the priorieties in preterm birth intervention.