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To investigate the role of catechol estrogens in human parturition, these steroids were analyzed in samples from the maternal venous and umbilical venous and arterial plasma at vaginal (n = 28) and abdominal (n = 28) delivery. To ensure the appropriateness of collection of umbilical artery and venous blood samples, progesterone content was also determined. Although there is no significant difference in maternal vein content of catechol estrogens between the two groups, the umbilical venous (p = 0.03) and arterial (p = 0.002) plasma concentrations are significantly higher at vaginal delivery than those measured at abdominal delivery. In view of the present data and the importance of catechol estrogens in prostaglandin synthesis and in potentiating the activity of catecholamines through competitive inhibition of catechol-O-methyltransferase, it is suggested that catechol estrogens may play a role in triggering the events involved in the onset of labor and delivery in humans.