During pregnancy, a new structure, the conceptus, arises de novo, grows, ages, and is ultimately expelled. In the maternal host, these awesome phenomena are accompanied by major metabolic alterations. It is the purpose of the present communication to focus upon one aspect, namely, the changes in maternal carbohydrate metabolism, and to summarize recent investigations concerning the possible role of the conceptus in their development. The Problem For the human female, pregnancy constitutes a form of diabetogenic challenge.* Although the disposition of intravenously administered glucose is normal,3 the decline in blood sugar following standard doses of exogenous insulin is subnormal.4 The dichotomy would suggest that preservation of normal glucose tolerance during pregnancy necessitates an enhanced secretion of endogenous insulin. This premise is supported by the gestational changes in the histology of the pancreatic islet cells.5,6 It is further reinforced by the clinical experience in subjects with marginal or