ObjectiveTo determine if labor activates the fetal fibrinolytic system.MethodsA total of 59 umbilical venous blood samples were collecting following vaginal delivery at term (n= 20), cesarean delivery following labor at term (n= 12), vaginal delivery before term (n= 18), and cesarean delivery without labor (n= 9). D-dimer concentrations, a sensitive marker of fibrinolysis, were measured by enzyme-linked immunosorbent assay, and compared between groups by Kruskel-Wallis and Mann WhitneyUtests, with significance defined asP< .05.ResultsThere were no significant differences in median D-dimer concentrations between newborns delivered vaginally or by cesarean after term labor or preterm labor. There were significant differences in median umbilical venous D-dimer concentrations in subjects delivered vaginally or by cesarean after term or preterm labor compared with term subjects without labor delivered by cesarean (427, 773, and 326 versus 87 ng/mL,P= .01)ConclusionElevation of umbilical plasma D-dimer concentrations in laboring patients suggests activation of fetal fibrinolysis before delivery.