The aim of this research was to study the concentration of urinary trypsin inhibitor in amniotic fluid (AF) and its effect on myometrial contraction in term and preterm deliveries. Urinary trypsin inhibitor was measured in AF of term and preterm labor. Immunohistochemical staining of amnion and myometrium was carried out. Isometric uterine contraction was studied to elucidate the effect of AF and urinary trypsin inhibitor on the contractile activity of term and preterm myometrium. The effect of urinary trypsin inhibitor on the production of prostaglandin E2 (PGE2) from myometrial cultures stimulated by IL-8, IL-1 and LPS was verified. Urinary trypsin inhibitor was significantly increased in AF of cases of preterm delivery (p < 0.0001). Amnion and myometrium of preterm deliveries were faintly stained for urinary trypsin inhibitor compared to term delivery. Amniotic fluid and urinary trypsin inhibitor could successfully inhibit-myometrial contraction. Also, urinary trypsin inhibitor could significantly inhibit the production of PGE2 in the myometrial cell cultures stimulated by IL-1 and LPS (p < 0.001 and 0.0005). IL-8 has no significant effect on PGE2 production from myometrial cell culture. Urinary trypsin inhibitor suppresses myometrial contraction in term and preterm deliveries. It may play a role in maintaining normal pregnancy and preventing preterm delivery.