Background: Half of women with overweight gain excessive weight during pregnancy. Perceived self-efficacy plays a significant role in adherence to healthy behaviours, especially in turning points of life, such as a pregnancy.

Objectives: To assess the effectiveness of the Supporting lifestyle change in pregnant mothers with obesity through the wearable internet-of-things (SLIM)-intervention in terms of improving self-efficacy in eating and physical activity and preventing excessive weight gain of pregnant women with overweight. The second aim was to evaluate the fidelity of the intervention implementation.

Methods: Pregnant women with overweight (N = 54) attending maternity clinics in Southwest Finland between 2021 and 2023, were recruited using convenience sampling. The SLIM-intervention included: health technology, motivational interviewing, feedback, and goal setting. The intervention was implemented in prenatal visits in maternity clinics continuing to 12 weeks postpartum. The change in self-efficacy was measured with Weight Efficacy Life-Style Questionnaire (score 0–180) and Self-Efficacy for Physical Activity Scale (score 5–25) with repeated measures in three timepoints. Data on weight were collected from patient records.

Results: The levels of self-efficacy were high throughout the study period and there were no significant changes in self-efficacy in eating (p = 0.650) or physical activity (p = 0.936). Most of the women (N = 9/10), whose gestational weight gain was within recommendations or less, managed to lose weight during the postpartum period (p < 0.001).

Conclusions: Although the intervention was not effective in improving self-efficacy, the importance of recommended gestational weight gain on better postpartum weight management was highlighted. A long follow-up time was valuable for examination of changes over time and understanding trends, patterns, and outcomes of the study.

Registration: The trial was registered at the Clinicaltrials.gov register platform (ID NCT04826861) on March 17th, 2021.