IMPORTANCE There is a well-described association between maternal diabetes mellitus and risk of congenital heart disease (CHD) in offspring. Although the clinical diagnoses of type 2 diabetes or gestational diabetes are strong risk factors for CHD, subclinical abnormalities of glucose and insulin metabolism are common within the general population and could also confer risk for CHD. We hypothesize that continuous measures of blood analytes related to maternal diabetes are related to odds of cardiac malformations. OBJECTIVE To explore the potential association of 2 different CHD phenotypes in offspring with maternal midpregnancy measures of glucose and insulin. DESIGN, SETTING, AND PARTICIPANTS Case-control study from a population-based cohort of 277 pregnant women in southern and central California carrying infants with tetralogy of Fallot (TOF) (n = 55), dextrotransposition of the great arteries (dTGA) (n = 42), or healthy infants without CHD (n = 180). Serum samples were collected from 2003 through 2007. The analysis was conducted from March through June 2015.MAIN OUTCOMES AND MEASURES Blood analytes related to maternal glucose metabolism were measured from random nonfasting second-trimester blood samples. We measured serum insulin levels by a validated radioimmunoassay, and we measured glucose levels. Multivariable logistic regression models estimated the association between these levels and case status.