Objective-Factors associated with increasing maternal triglyceriDe concentrations in late pregnancy incluDe gestational age, obesity, preeclampsia, and altered glucose metabolism. In a subgroup of women in the Metformin in Gestational Diabetes (MiG) trial, maternal plasma triglycerides increased more between enrollment (30 weeks) and 36 weeks in those treated with metformin compared with insulin. The aimof this study was to explain this finding by examining factors potentially related to triglycerides in these women. Research design and methods-Of the 733 women randomized to metformin or insulin in the MiG trial, 432 (219 metformin and 213 insulin) had fasting plasma triglycerides measured at enrollment and at 36 weeks. Factors associated with maternal triglycerides were assessed using general linear modeling. ResultsMean plasma triglyceriDe concentrations were 2.43 (95% CI 2.35-2.51) mmol/L at enrollment. Triglycerides were higher at 36 weeks in women randomized to metformin (2.94 [2.80-3.08] mmol/L; +23.13% [18.72-27.53%]) than insulin (2.65 [2.54-2.77] mmol/L, P = 0.002; +14.36% [10.91-17.82%], P = 0.002). At 36 weeks, triglycerides were associated with HbA1c (P = 0.03), ethnicity (P = 0.001), and treatment allocation (P = 0.005). In insulin-treated women, 36-week triglycerides were associated with 36-week HbA1c (P = 0.02), and in metformintreated women, they were related to ethnicity. Conclusions-At 36 weeks, maternal triglycerides were related to glucose control in women treated with insulin and ethnicity in women treated with metformin. Whether there are ethnicity-related dietary changes or differences in metformin response that alter the relationship between glucose control and triglycerides requires further study.