Women with gestational diabete (GD) who do not meet glycemic control objectives with diet will be assigned to two treatment groups randomly. One: metformin at a dose of 850-2550mg every 24h; two: insulin detemir associated or not with rapid insulin analogue (aspart) according to your glycemic controls. The Metformin group may additionally receive insulin in a second time in case the glycemic control is not appropriate with monotherapy.

Women with gestational diabete (GD) who do not meet glycemic control objectives with diet will be assigned to two treatment groups randomly. One: metformin at a dose of 850-2550mg every 24h; two: insulin detemir associated or not with rapid insulin analogue (aspart) according to your glycemic controls. The Metformin group may additionally receive insulin in a second time in case the glycemic control is not appropriate with monotherapy.

The objectives are: Demonstrate that treatment with metformin in women with GD (not controlled with diet) can get no lower obstetric and perinatal outcomes than those with standard treatment with insulin.

Demonstrate that glycemic control with metformin in properly selected women, can be equivalent to that obtained with insulin.

Inclusion Criteria:

1. 18-45 years old.
2. Diagnosis of GD, with fasting glucose <120 mg / dL.
3. not controlled by diet: fasting capillary blood glucose> 95 mg / dl in at least 2-3 times or 1 hour postprandial >140 mg / dl on, at least 2-3 times a week.
4. 2nd or 3rd trimesters of pregnancy.
5. Able to give informed consent.

Exclusion Criteria:

1. Psychopathological situations that do not guarantee proper adhesion to follow up
2. 1st trimester of pregnancy
3. gastrointestinal diseases that may cause poorer tolerance or increased symptoms with metformin.
4. Patients who can not attend the scheduled consultation.
5. Language barrier limiting for understanding treatment settings
6. Twin pregnancy.