Trial halted early due to limited ability to recruit participants

This study is a prospective, unmasked randomized clinical trial comparing the use of insulin vs combination insulin and metformin for treatment in women diagnosed with gestational diabetes mellitus (GDM). The investigator's hypothesis is that the combination of metformin and insulin will be superior to insulin alone to achieve tight glucose control during pregnancy.

The objective of this study is to compare the effectiveness of insulin alone vs the combination of insulin and metformin in treating patients with gestational diabetes (GDM). Currently, outside of pregnancy, the treatment of type 2 diabetes mellitus (T2DM) with both metformin and insulin is superior to using insulin alone. In pregnancy, insulin alone has traditionally been used, though some advocate the use of metformin alone as primary therapy. There have been no trials published to date specifically comparing combination therapy to insulin alone. Our hypothesis is that the combination of metformin and insulin will improve overall control of blood glucose, the improvement of which has been demonstrated to improve maternal and neonatal outcomes. Control of blood glucose will be determined by hemoglobin A1c at the time of delivery.

Inclusion Criteria:

• Able to read and write English and/or Spanish and give written consent
• Diagnosis of GDM, defined as an abnormal glucose tolerance test performed after 12 weeks gestation using 1 of the 2 criteria below:
• 50 gram 1 hour oral diabetes screening testing yielding a result of > 200 mg/dL
• A 100 gram 3 hour oral glucose tolerance testing yielding >2 abnormal values (normal values defined as fasting blood glucose < 95, 1 hour < 180, 2 hour < 155 and 3 hour < 140)
• Singleton gestation
• Gestational age between 12 and 34 weeks and 6 days determined by last menstrual period (LMP) confirmed by ultrasound using criteria set forth by the ACOG (Committee on Obstetric Practice). If LMP is unknown then gestational age must be set by ultrasound prior to 20 weeks gestation.

Exclusion Criteria:

• Pre-existing DM either by diagnosis preceding pregnancy or hemoglobin A1c >6.5 collected during the current pregnancy
• Uncontrolled chronic hypertension, as this may alter maternal and perinatal outcomes measured.
• Multiple gestations
• Major fetal anomalies anticipated to require NICU admission
• Contraindication to metformin (allergy, history of lactic acidosis, pre-existing renal disease (Cr >1.5 mg/dL), active liver disease, current alcohol abuse).
• Vitamin B12 deficiency, as metformin reduces intestinal absorption of vitamin B12
• Medications known to effect glucose metabolism other than insulin and metformin as this may mask any effect between the two treatments.
• Known inability to tolerate metformin.