Normal gestation is associated with a gradual change in lipid, bile acid and glucose metabolism that results in relative hypertriglyceridaemia, hypercholesterolaemia, hypercholanaemia and impaired glucose tolerance by late pregnancy. Endocrine signals are known to contribute to this gestational switch, and it is likely to be an adaptation to ensure adequate nutrient transfer to the developing embryo. In susceptible women, the gestational alterations in metabolism can be more marked resulting in diseases, e.g. gestational diabetes mellitus or intrahepatic cholestasis of pregnancy. The gut microbiota contribute to bile acid metabolism via deconjugation and enzymic modification of luminal bile acid species. It has been shown that transplantation of faeces from women in the third trimester of uncomplicated pregnancy to germ free mice results in enhanced weight gain and insulin resistance compared to transplantation of samples from women in the first trimester of pregnancy. There is an alteration in gut microbes in normal gestation, with modification of the bile acid pool and entero-endocrine signals that influence bile acid and lipid metabolism. The lecture will focus on the impact of these alterations and the relevance to gestational metabolic switching in health and disease.