Yes, sugars (carbohydrates) should be avoided and a thorough medical control is essential throughout pregnancy in these cases. It is important to eat foods with a lot of fiber (fruits, vegetables, whole grain breads, cereals, legumes), as they are absorbed more slowly than simple carbohydrates and help blood sugar levels not to rise too quickly after meals.
The doctor or dietician will be in charge of indicating to the woman the foods she should eat and adapting them to the nutritional needs at each stage of pregnancy.