Gestational diabetes mellitus (GDM) is one of the most common complications during pregnancy. It can lead to physiological reactions that can increase the incidence of preeclampsia and infections, the risk of postpartum bleeding in the short term, and the possibility of conversion Type II diabetes in the long term.The present study intended to determine the effects of diaphragmatic breathing exercises (DBE) on physiological parameters in pregnant women with GDM.The research team designed a single-blinded, randomized, controlled clinical trial.The study was performed at a university hospital in Istanbul, Turkey.Participants were 60 pregnant women who suffered from GDM and who attended the antenatal polyclinic at the hospital between June 2015 and June 2016.Participants were divided randomly into 2 groups with 30 participants each, the intervention and control groups. The intervention group practiced DBE for 5 minutes every day; the control group did not. Diet therapy has been recommended to both groups by their obstetrician.The outcome measures were the levels of fasting blood glucose (FBG), and vital measurements were taken from both groups for 30 days. Data analysis was done using SPSS statistical software. Results • The experimental group showed significant decreases in respiratory rate and levels of FBG when compared with the control group. The intervention group's results for pulse rate and systolic and diastolic blood pressure were within normal limits, and no measurable differences existed between the two groups for those measures.Practicing DBE provided positive effects for pregnant women with gestational diabetes mellitus on the observed physiological parameters.