The aim of this randomized controlled trial is to determine the effect of probiotic yoghurt (containing Lactobacillus Acidophilus LA5 and Bifidobacterium Lactis BB12) on maternal blood glucose (primary outcome), as well as maternal and neonatal outcomes (secondary outcomes). After signing written informed consent, 130 pregnant women at about 20 weeks of gestation, covered by health center in Tabriz-Iran, with BMI≥25 and normal fasting blood sugar level will be recruited and allocated into two groups using stratified (overweight or obese) block randomization with allocation ratio of 1:1. Consequently numbered opaque sealed envelopes containing a paper with a unique group code will be used to conceal the allocation. The allocation sequence and envelops will be prepared by a person not involved in data collection. Participants will be asked not to consume any probiotic products for two weeks and consume 100 g probiotic or placebo yogurt daily, identified with the group code, from 24th weeks of gestation until delivery. At 28 weeks of pregnancy, fasting blood glucose and glucose tolerance test; at 37 weeks of pregnancy, medical complications like preterm delivery, preeclampsia, as well as maternal physical activity; and following delivery, maternal (preterm delivery, preeclampsia, pregnancy weight gain and gestational diabetes) and neonatal (birth weight, hyperbilirubinemia and neonatal death) outcomes will be examined. Compliance rate, satisfaction and side effects of probiotics will also be determined.