Experimental Approaches

Much insight into the maternal fetal relationship was gained from acute experiments on anesthetized pregnant ewes by Joseph Barcroft and his successors during the first half of the 20th century.[1] However, assessment of normal fetal responses to changes in maternal physiology and metabolism did not become possible until the 1960s when Barron's group developed surgical and experimental techniques for chronic vascular catheterization and instrumentation of the sheep fetus in conscious intact ewes.[2] Simultaneous measurement of uterine and umbilical blood flows and concentrations of gases, nutrients, and metabolites in relevant arteries and veins allowed quantitative assessment of net exchanges of these materials between the maternal and fetal circulations. Net metabolism in nonfetal (predominantly placental) conceptus tissues was calculated as the difference between net uterine and net umbilical exchange of a substrate (Fig. 1). Application of isotope dilution tracer techniques added further precision to the measurement of maternal fetal exchanges and, by enabling measurement of rates of maternal production and utilization of nutrients, provided a means of estimating the partition of nutrients between maternal and conceptus tissues[3] (Fig. 1).

These experimental approaches to investigation of maternal fetal relationships have been applied most extensively to sheep, but have also been used successfully in other farm animal species, including cattle, horses, and

pigs.

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