Fetal growth in utero depends on nutrient and oxygen reaching the fetus through the uterine and placental microcirculations, both undergoing massive expansion during pregnancy. Aberrations of the placental vasculature are associated with Intrauterine Growth Restriction (IUGR), a common pathological outcome of pregnancy; however, the cellular components responsible for vessel formation in the placenta and the uterus remain unknown.Endothelial Progenitor Cells (EPC) are a group of morphologically and functionally varied bone marrow derived vasculogenic cell types, divided into two major subsets: (i) Circulating Angiogenic Cells (CACs), which promote vessel formation by interfering with the extracellular matrix and (ii) Endothelial Colony Forming Cells (ECFCs), which provide the source for new endothelium. This role has been demonstrated in pathophysiological studies, but not in normal physiological events in vivo. Fetal ECFCs are more proficient than their adult counterparts, but it is unclear in what specific fetal or maternal physiological situations fetal ECFCs are involved. Based upon these considerations, it was hypothesised that: (i) fetal-derived ECFCs play a role in placental vasculogenesis, (ii) these cells transmigrate the placenta and home to loci of vessel formation in the pregnant uterus, and that (iii) intrinsic alterations in their capabilities are associated with fetal growth restriction during intrauterine life.To support these hypotheses the following experiments were performed;(i) EPCs in blood from pairs of human umbilical arteries and veins were counted by flow cytometry. Numbers of EPCs in these samples showed an arterio-venous gradient suggesting their placental sequestration. Furthermore, ECFCs were isolated from human umbilical blood using established culture techniques. Labelled human fetal ECFCs were transplanted into the circulation of murine fetuses using an ultrasound-guided intra-cardiac injection. Using a fluorescent imager and microscopy these cells were shown to home to the murine placenta and participate in vasculogenesis.(ii) Male mice ubiquitously expressing eGFP were crossbred with native females, and fetal (eGFP-positive) endothelial-like cells integrated into the uterine microvasculature. Human fetal ECFCs injected into murine fetuses were shown to migrate to the maternal uterus and became functionally involved with the microvasculature. In humans, microvessels were isolated from uterine biopsies of mothers with male offspring. Copies of the male specific SRY gene (quantified by RT-QPCR) indicated that cells of fetal origin constituted 12% of the endothelium in these vessels. In cross-sections, hybridisation of the Y-chromosome demonstrated the presence of fetal cells in the maternal endothelium of the human uterus. (iii) Using flow cytometry, fewer EPCs were defined within the peripheral circulation of growth-restricted babies. Functional assays showed that ECFCs derived from these growth-restricted cases had intrinsically impaired proliferation, migration, matrix-metalloproteinase (MMP-2) production, and generated fewer blood vessels in a murine vasculogenic bioassay.These results demonstrated the vasculogenic capacity of human fetal ECFCs in vivo and established them as key players in human placental vasculogenesis and uterine vessel expansion. Notably, these results also showed a link between impaired function of fetal ECFCs and IUGR, which is associated with increased cardiovascular risk of both the fetus as an adult, and mother in later life. From these findings it could be speculated, that intrinsic changes in ECFC-biology may be the causative link between IUGR and fetal and maternal cardiovascular susceptibility. Insight into these processes may contribute to early diagnosis, prevention and treatment of IUGR and associated conditions.