• A composite of: pregnancy loss, preterm birth, birth injury, respiratory distress, neonatal hypoglycemia, and NICU admission > 48 hours. [ Time Frame: conception to 28 days after birth ] [ Designated as safety issue: Yes ]
  

• Large for gestational age (LGA) infants [ Time Frame: Up to 24 hours after birth ] [ Designated as safety issue: Yes ]
  Defined as greater than the 90th percentile for weight, based on the National canadian fetal growth standards for singleton boys and girls.
  
  
• Pregnancy loss [ Time Frame: Up to 40 weeks gestation ] [ Designated as safety issue: Yes ]
  Spontaneous abortion defined as death of a fetus at <20 weeks gestation; Stillbirth defined as death of a fetus with a birth weight ≥ 500g or at ≥ 20 wks gestational age regardless of birth weight; Neonatal death defined as death of a live born infant within the first 28 days of life or prior to hospital discharge, whichever is later.
  
  
• Preterm birth [ Time Frame: Up to 37 weeks gestation ] [ Designated as safety issue: Yes ]
  Birth < 37 weeks gestation
  
  
• Respiratory distress [ Time Frame: within 72 hours after birth ] [ Designated as safety issue: Yes ]
  Given surfactant via an endotracheal tube and/or requiring assisted positive pressure ventilation within 72 hours after birth.
  
  
• Neonatal hypoglycemia [ Time Frame: NICU admission >24 hours ] [ Designated as safety issue: Yes ]
  A plasma gluclose <2.6 mmol/L on one or more occasions, starting at 30-60 minutes after birth, and necessitating intravenous dextrose within the first 48 hours of life.
  
  
• NICU admission >24 hours [ Time Frame: NICU admission >24 hours ] [ Designated as safety issue: Yes ]
  Admission to a neonatal intensive or special care unit for > 24 hours during the initial hospitalization after birth
  
  
• Cord blood gases <7.0 [ Time Frame: Within 4 hours of birth ] [ Designated as safety issue: Yes ]
  
• Hyperinsulinemia as measured by elevated cord blood C-peptide [ Time Frame: Within 4 hours of birth ] [ Designated as safety issue: No ]
  A cord serum C-peptide value > 1.7 ug/L (which is >90th percentile of values for the total cohort of participants in the HAPO trial) will be defined as hyperinsulinemia.
  
  
• Maternal glycemic control as measured by HbA1c and capillary glucose measurements. [ Time Frame: Up to 40 weeks gestation ] [ Designated as safety issue: Yes ]
  Gestational age at testing will be recorded. All downloaded glucose results will be transmitted on a regular basis to a central site for future analysis. Monthly correlations will be done with the laboratory during routine monthly blood draws.
  
  
• Maternal hypoglycemia [ Time Frame: Up to 40 weeks gestation ] [ Designated as safety issue: Yes ]
  Maternal hypoglycemia defined as mild (<3.6, symptomatic and asymptomatic or requiring treatment), or severe (loss of consciousness or confusion requiring assistance) will be documented at each visit.
  
  
• Maternal weight gain [ Time Frame: Up to 40 weeks gestation ] [ Designated as safety issue: Yes ]
  The first and last weight will be obtained at the first and last visit in pregnancy, whether they be done by the endocrinologist, family physician or obstetrician.
  
  
• Maternal insulin doses [ Time Frame: Up to 40 weeks gestation ] [ Designated as safety issue: Yes ]
  Maternal insulin doses (overall amount and number of patients that are taking 'high' insulin doses defined as 2 Units/kg or more per day)
  
  
• Pre-eclampsia, and/or gestational hypertension [ Time Frame: Up to 40 weeks gestation ] [ Designated as safety issue: Yes ]

  Gestational hypertension: New onset of hyperstension (diastolic of 90 mm Hg or higher) in pregnancy after 20 weeks gestation in a woman with previously normal blood pressure (124, 125).

  Pre-eclampsia: new-onset proteinuria, Eclampsia (Seizures in pregnancy), Elevated liver function tests, Decreased platelet count < 100 x 109/L, Elevated serum creatinine (> 80 mmol/L) or small for gestational age infant (birth weight less than 10%).

  
  
• Sepsis [ Time Frame: Up to 28 days after birth ] [ Designated as safety issue: Yes ]
  A positive blood and/or cerebral spinal fluid culture during the neonatal hospital stay.
  
  
• Hyperbilirubinemia [ Time Frame: First 7 days of life ] [ Designated as safety issue: Yes ]
  Significant jaundice was present based on bilirubin levels requiring treatment with phototherapy> 6 continuous hours, or an exchange transfusion, or receiving intravenous gamma globulin, or requiring readmission into hospital during the first 7 days of life.
  
  
• Number of hospitalizations [ Time Frame: Up to 40 weeks gestation ] [ Designated as safety issue: No ]
  Number of hospitalizations prior to admission for delivery and the duration of hospital stays for the mother prior to admission for delivery and associated with delivery.
  
  
• Rate of caesarean-section [ Time Frame: Up to 40 weeks gestation ] [ Designated as safety issue: Yes ]
  
• Duration of hospital stay for infant. [ Time Frame: Up to 28 days after birth ] [ Designated as safety issue: No ]
  Duration of hospital stay for infant associated with his/her birth until the first discharge home
  
  
• Fetal fat mass [ Time Frame: Up to 7 days after birth ] [ Designated as safety issue: Yes ]
  Fetal fat mass compared with women treated with insulin plus placebo
  
  
• Birth Injury [ Time Frame: Up to 7 days after birth ] [ Designated as safety issue: Yes ]
  Defined as any of the following: spinal cord injury, basal skull fracture or depressed skull fracture, clavicular fracture, long bone fracture, subdural or intracerebral hemorrhage or any kind
  
  

Type 2 diabetes in pregnancy is increasing in prevalence and these women continue to face increased rates of adverse maternal and fetal outcomes. The investigators hypothesize that metformin use, as an adjunct to insulin, will decrease these adverse outcomes by reducing maternal hyperglycemia, high maternal insulin doses, excessive maternal weight gain and gestational hypertension/pre-eclampsia, all of which should reduce perinatal and neonatal mortality and morbidity. In addition, since metformin crosses the placenta, metformin treatment of the fetus may have a direct beneficial effect on neonatal outcomes. This study is an randomized controlled trial (RCT) that adds metformin to insulin, and is a double-blind, placebo-controlled RCT. The investigators believe that neither metformin alone nor insulin alone will effectively treat this population, and therefore our design, which includes the addition of metformin to insulin, will be the most relevant to our patients.