Figure 3 Metabolic and growth responses to a three day maternal fast in late gestation twin fetuses of ewes that were either well nourished or undernourished from 60 days before to 30 days after mating. The experiment included 10 well nourished and nine undernourished twin pairs; this figure is a representative example of the responses seen in the groups as a whole. (A) Fetal arterial glucose concentrations; (B) fetal arterial lactate concentrations; (C) fetal arterial oxygen tension (Pao₂), and (D) increment in fetal chest girth. The duration of the maternal fast is indicated by the solid black bar; term  =  145 days. Note the complex interaction in fetal glucose and lactate responses during the late‐gestation fast between maternal periconceptional nutritional status and twin growth status. Before the late‐gestation fast, the undernourished twin pair have higher glucose and lactate concentrations, consistent with use of lactate as an alternative fuel source; during the maternal fast, concentrations of glucose fall and lactate rise, but the lighter twin of the periconceptionally well nourished ewe has the highest, and the lighter twin of the periconceptionally undernourished ewe the lowest, lactate and glucose concentrations. During the maternal fast, the heavier twin of the periconceptionally well nourished ewe maintains its growth rate and there is no change in Pao₂; in the other three fetuses growth rate slows and Pao₂ rises, consistent with reduced oxidative demand.