Background and Objective: Pregnancy is a period of women’s life which is usually associated with reducing maternal daily physical activity. However, maintaining adequate daily physical activity is important for mother and fetus. This study was conducted to determine the relationship between exercise before and during pregnancy and gestational diabete mellitus (GDM).

Methods: This case-control study was conducted on 80 pregnant women with gestational diabete mellitus and 57 non-diabetic pregnant women. Physical activity during one and eight years before, and first and second trimesters of pregnancy was recorded through interview.

Results: Regular participation in exercise activities in non-diabetic women were significantly more than diabetic women during all periods (P<0.05). Regular participation in exercise during one year before and second trimester of pregnancy was inversely correlated to GDM (P<0.05). Energy expenditure of second trimester of pregnancy was significantly correlated to reduce GDM (P<0.05).

Conclusion: Regular exercise before and during pregnancy can be effective in reducing the probability of GDM. Exercise during one year before and especially second trimester of pregnancy are more important predictors of GDM.