Gestational diabetes mellitus (GDM) is one of the most widespread medical problems in pregnancy. The most common medication is insulin, which requires daily injections; another option is metformin, which is an oral pill. The purpose of this study is to determine whether metformin works as well as insulin to prevent pregnancy complications. Researchers also want to learn whether metformin is as safe as insulin for mothers and children (who were born to mothers who received treatment). Additionally, the study will examine the reasons (if any) why pregnant people with GDM prefer metformin versus insulin. Study procedures include completion of short surveys, blood sample collection, and medical records review (including how well the participant and newborn baby are doing during and after delivery).