Context: Gestational diabetes mellitus (GDM) stands out as a prevalent health issue during pregnancy. Health coaching emerges as a technique that enhances the well-being and health outcomes of women grappling with gestational diabetes. Aim and design: A pre and post-test quasi-experimental study design was employed, aiming to evaluate the effect of a lifestyle coaching program on health-related behavior and blood glucose level among gestational diabetic women. Sample: A purposive sample of sixty pregnant women was chosen from Kafr El-Sheikh University Hospital's prenatal outpatient clinic, which served as the study's setting. Tools: In the current study, four tools were used: a structured interviewing questionnaire, a gestational diabetes knowledge assessment questionnaire, a prenatal health behavior scale, a self-record or diary, and an Arabic educational booklet was used as supportive material. Results: The current study found that, when comparing the pre- and post-coaching program implementation periods for the studied sample, there was a highly statistically significant improvement in the total knowledge score and the total mean of health-promoting behavior regarding gestational diabetes mellitus (P<0.001). Additionally, the total mean of health-impairing behavior related to gestational diabetes as well as the total mean of fasting and postprandial blood glucose levels were significantly lower at post-coaching program implementation than they were at pre-program implementation (P < 0.001). Conclusion: The implementation of a lifestyle coaching program has a better effect on pregnant women's knowledge and health-related behavior regarding gestational diabetes mellitus, besides decreasing women's blood glucose level. Recommendations: The Lifestyle Coaching Program ought to be incorporated into the prenatal routine care for pregnant women with gestational diabetes in the various health care settings