There is evidence that controlling total amount of carbohydrates is a strategy for controlling glucose levels in diabetes mellitus. There is not major evidence that any given macronutrient percentage may be recommended to treat a woman with Gestational Diabetes Mellitus (GDM). In the investigators' country, insulin is the second-line treatment once medical nutrition therapy (MNT) has failed to control glucose levels during pregnancy. Insulin treatment is more expensive and not as well accepted as MNT. The investigators have designed a randomized-controlled trial to assess whether a diet with 40% total calories from carbohydrates may reduce the need of insulin treatment in women with gestational diabetes, without having unfavourable pregnancy outcomes, in comparison with a diet with 55% total calories from carbohydrates.

Inclusion Criteria:

• Gestational diabetes mellitus
• Single pregnancy

Exclusion Criteria:

• Multiple pregnancy
• High risk pregnancy (any other concomitant illness, excluding hypertension and obesity)
• Low ability to understand and follow a diet
• Need to use insulin different from regular insulin