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This chapter reviews the roles and distribution of O2 inside the early human gestational sac and its impact on our understanding of the physiology of O2 metabolism by the fetus during the first and early second trimesters of pregnancy. The mammalian fetus is exposed to major fluctuations in O2 concentration from conception to delivery. The first trimester gestational sac has additional barriers to materno-fetal exchange compared to the definitive placenta of the second and third trimesters of pregnancy. Ample dilation of the uteroplacental circulation together with rapid villous angiogenesis are the key factors necessary to adequate placental development and function and subsequent fetal growth. Overall the data have confirmed that the human placenta is not truly hemochorial until the end of the first trimester. Rather it is deciduochorial, being supported by tissue fluids and endometrial secretions.
The relationship between assisted reproductive technologies (ART) and deregulation of genomic imprinting is thought to have particular relevance to fetal growth because fetal growth is suggested to be the main drive for the evolution of imprinting or the conflict between the parental genomes. The link between embryo culture and phenotype is thought to be, at least in part, epigenetic. Genomic imprinting is an important epigenetic mechanism through which some aspects of fetal growth and development are regulated. Imprinting is controlled by allelic methylation, and the setting up of imprints in the female germline is a process that occurs throughout reproductive life. Inappropriate methylation or expression of imprinted gene clusters cause a range of pathologies, often involving aberrant growth and development. Similarly, the use of ART has been shown to cause disorders of growth in humans and other mammals.
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