Objective: The purpose of this study was to determine whether expressions of the cell adhesion molecules LFA-1 (CD11a), VLA-4 (CD49d), and L -selectin (CD62L ) on CD34⁺ stem and progenitor cells in umbilical cord blood change during gestation. Study Design: In a prospective observational study 3-color fluorescence-activated cell sorting was used to assess the levels of expression of CD11a, CD49d, and CD62L on CD34⁺ cells in fresh cord blood samples collected at delivery between 22 and 42 weeks’ gestation. Results: The relative number of CD34⁺ cells decreased as gestational age increased (r = –0.71; P <.001). Conversely, we found significant increases in cell adhesion molecule expression by CD34⁺ cells during gestation (LFA-1, r = 0.47; P =.001; VLA-4, r = 0.33, P =.031; L -selectin, r = 0.61; P <.001). Comparisons between grouped samples from early preterm (22-32 weeks’ gestation), late preterm (33-37 weeks’ gestation), and term (38-42 weeks’ gestation) infants confirmed this correlation and revealed that the major increases occurred between early and late preterm gestation. Conclusion: These results suggest a role for cell adhesion molecule expression in the process of migration and homing of circulating stem cells to the fetal bone marrow toward the end of pregnancy. The findings may have implications for the use of preterm cord blood for hematopoietic stem cell transplantation and also for prenatal gene therapy. (Am J Obstet Gynecol 2000;183:1152-7.)