BACKGROUND: The aim of this study was to determine the influence of mode of birth and umbilical cord blood (CB) collection before (in utero) or after delivery of the placenta (ex utero) on total number of WBCs and CD34+ cells in CB units. STUDY DESIGN AND METHODS: Consecutively donated, banked CB units were assessed for net volume, WBC concentration, total number of WBCs, proportion of CD34+ cells, and total number of CD34+ cells. These parameters were then correlated with the mode of birth and the mode of CB collection relative to the delivery of the placenta. RESULTS: A significantly higher CB volume was seen following cesarean section (n = 61) than following vaginal delivery (n = 157; median volume, 76 vs. 63 mL, respectively; p < 0.0001). In contrast, CB from vaginal delivery had a significantly higher WBC concentration compared with CB from cesarean section (medians, 17.1 x 10(9) and 13.6 x 10(9) WBCs/L, respectively; p < 0.0001). The mode of birth did not influence the proportion of CD34+ cells. A correlation was demonstrated between the total number of CD34+ cells and the total number of WBCs. As a consequence of the opposing effects on volume and WBC counts by cesarean section and vaginal delivery, there were no significant differences in the total number of WBCs or CD34+ cells for the CB units with mode of delivery in this study. No significant differences were found in CB with mode of CB collection (in utero [n = 58] or ex utero [n = 99]) following vaginal delivery. CONCLUSIONS: The mode of birth influences the CB WBC concentration and volume collected and should be taken into consideration for establishing any acceptance limits for CB units to be banked. There were no differences in CB with in utero or ex utero collections.