Background We investigated the roles of inflammatory cytokines and the A disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) family in the etiopathogenesis of spontaneous preterm delivery by comparing the ADAMTS4, ADAMTS5, interleukin (IL)-6, and tumor necrosis factor-α (TNF-α) levels in second-trimester amniotic fluid between pregnant women with preterm birth and term controls. Methods All pregnant women who underwent second-trimester amniocentesis for genetic analysis between January 1, 2016, and January 1, 2018, were enrolled in this study. From this cohort, 22 patients who subsequently experienced spontaneous preterm delivery before 34 weeks of pregnancy formed the study group, and 22 age- and body mass index (BMI)-matched patients without preterm birth constituted the control group. Results No significant differences were observed between the preterm birth and control groups in terms of age, BMI, obstetric history of preterm delivery, gestational age at amniocentesis, or indication for amniocentesis. The mean amniotic fluid levels of ADAMTS4 and ADAMTS5 were significantly increased in the preterm birth group compared to the control group (248.3±22.6 and 182.4±19.8 pg/mL, P=0.012; and 198.6±21.6 and 159.1±21.7 pg/mL, P=0.035, respectively). Significantly increased IL-6 and TNF-α levels were also detected in the amniotic fluid of women who experienced spontaneous preterm delivery, relative to controls (142.1±16.2 and 95.8±16.4 pg/mL, P<0.001; and 139.4±12.5 and 89.6±11.2 pg/mL, P<0.001, respectively). Conclusion The results of this study imply that increased mid-trimester amniotic fluid levels of ADAMTS4, ADAMTS5, IL-6, and TNF-α play an important role in the pathophysiology of spontaneous preterm delivery.