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Maternal metabolism changes substantially during pregnancy. Early gestation can be viewed as an anabolic state in the mother with an increase in maternal fat stores and small increases in insulin sensitivity. Hence, nutrients are stored in early pregnancy to meet the feto-placental and maternal demands of late gestation and lactation. In contrast, late pregnancy is better characterized as a catabolic state with decreased insulin sensitivity (increased insulin resistance). An increase in insulin resistance results in increases in maternal glucose and free fatty acid concentrations, allowing for greater substrate availability for fetal growth.
Lain et al. (2007) studied this question.