Abnormal glucose metabolism in pregnancy is a spectrum. This spectrum stretches from mild forms of glucose intolerance that do not rise to the level of diabetes, to diabetes that first occurs in pregnancy, as well as to pregravid forms of diabetes associated with end-organ disease. In this review, we first discuss risk factors common to all forms of abnormal glucose metabolism in pregnancy. A review of how abnormal glucose metabolism in pregnancy is diagnosed precedes discussion of perinatal risks associated with different degrees of glycemic aberration. We discuss how to intervene in the ambulatory setting to mitigate these risks.