This study is designed as a single-blind parallel-group randomized controlled trial to develop a mobile health application based on the Information-Motivation-Behavioral Skills (IMB) Model and Social Cognitive Theory for gestational weight management and to examine the effect of the mobile application on weight management during pregnancy. The primary aim is to achieve gestational weight gain following the Institute of Medicine (IOM) guidelines. Secondary aims include improving attitudes towards healthy eating during pregnancy, increasing physical activity levels, and enhancing maternal (pregnancy, childbirth, and postpartum period) and fetal (weight, etc.) health outcomes.

The hypotheses of the study are as follows:

1. H1 Hypothesis: The intervention group using the mobile application will have a higher rate of gestational weight gain by IOM guideline recommendations compared to the control group (One-tailed H1 hypothesis).
2. H1 Hypothesis: The intervention group using the mobile application will have a higher score on the healthy eating attitude scale compared to the control group (One-tailed H1 hypothesis).
3. H1 Hypothesis: The intervention group using the mobile application will have a higher level of physical activity compared to the control group (One-tailed H1 hypothesis).
4. H1 Hypothesis: The intervention group will have a higher score on the healthy eating attitude scale after using the mobile application compared to before (One-tailed H1 hypothesis).
5. H1 Hypothesis: The intervention group will have a higher level of physical activity after using the mobile application compared to before (One-tailed H1 hypothesis).
6. H1 Hypothesis: Women with appropriate gestational weight gain will have a lower perinatal risk rate than women with excessive gestational weight gain (One-tailed H1 hypothesis).

Participant Population:

• Age between 18-45 years
• Singleton pregnancy
• Literate in Turkish
• Gestational age ≤ 12 weeks
• Pre-pregnancy/pregnancy Body Mass Index (BMI) between 18.5-39.9 kg/m² (normal, overweight, and obese individuals)
• Ownership of a smartphone

During the data collection phase, pregnant women will first fill out the Enrollment Criteria and Preliminary Registration Form to create their records, determining included and excluded participants. After preliminary tests, participants will be divided into intervention and control groups using stratified randomization based on their BMI. Participants will be stratified into normal BMI, overweight, and obese BMI categories. Pregnant women who meet the inclusion criteria will be given general information about the study and their written and verbal consent will be obtained. Pregnant women will be included in the study by the 12th week of pregnancy and/or earlier, and after their information and preliminary tests are collected, those in the intervention group will start using the mobile application when they reach the 12th week of pregnancy.

Data for the study will be collected using the Pregnancy Introduction Form, which includes sociodemographic information about the pregnant women, the Follow-up Form, which includes information from interim and final follow-ups, the Healthy Eating Attitude Scale developed to measure the attitude towards healthy eating, and the International Physical Activity Questionnaire to determine physical activity levels. Interim follow-ups of pregnant women in both groups will be conducted at the 24th week of pregnancy, during which the Follow-up Form, Healthy Eating Attitude Scale, and International Physical Activity Questionnaire will be administered, and current weight and height will be recorded. Similarly, the final follow-up will be conducted during the last prenatal check-up (38th week of pregnancy), where the same forms and measurements will be repeated. At the end of the study, to objectively evaluate participants in the intervention group, the User Version of the Mobile Application Rating Scale (uMARS) will be applied within the first week after birth to evaluate the mobile application.

The control group will receive routine care following the Ministry of Health's Prenatal Care Management Guide of the Republic of Turkey. To enhance adherence among control group participants, weekly antenatal health information unrelated to healthy eating, physical activity, and weight management will be provided via phone. Interim follow-ups will be conducted face-to-face during routine check-ups (for OGTT) at the 24th week of pregnancy, during which the Follow-up Form, Healthy Eating Attitude Scale, and International Physical Activity Questionnaire-Short Form will be administered, and current weights will be measured with a standard simple scale brought by the researcher (same scale) and recorded. The final follow-up will be conducted face-to-face during the last prenatal check-up at the 38th week of pregnancy, where the Follow-up Form, Healthy Eating Attitude Scale, and International Physical Activity Questionnaire-Short Form will be administered again, and current weights will be measured (with the same scale).

In addition to the routine care provided according to the Turkish Ministry of Health's Prenatal Care Management Guidelines, the intervention group will receive a mobile application intervention. The intervention (mobile application) will begin in the 12th week of pregnancy. Participants in the intervention group will start using the mobile application at the 12th week of pregnancy and continue to use it throughout their pregnancy. The content of the mobile application includes information on healthy nutrition and exercise during pregnancy, as well as monitoring and notifications. Pregnant women will be guided, encouraged, and monitored for healthy nutrition and exercise through the mobile application.

At the 24th week of pregnancy, interim assessments will be conducted for the participants in the intervention group. These assessments will include the Tracking Form, the Attitude Scale Towards Healthy Nutrition, and the Short Form of the International Physical Activity Questionnaire, administered face-to-face during routine check-ups. The current weight of the participants will be measured using a standard scale (the same scale) brought by the researcher, and the data will be recorded.

The final assessment for the group will be conducted during the last prenatal check-up (at the 38th week of pregnancy), where the Tracking Form, the Attitude Scale Towards Healthy Nutrition, and the Short Form of the International Physical Activity Questionnaire will be administered face-to-face, and the current weight will be measured using the same scale.

At the end of the study, within the first week after childbirth, participants in the intervention group will be asked to evaluate the mobile application using the User Version of the Mobile Application Rating Scale (uMARS).

Inclusion Criteria:

• Aged between 18 and 45 years
• Singleton pregnancy
• Able to read and write in Turkish
• Pregnant women with a gestational age ≤ 12 weeks
• Pre-pregnancy/starting BMI between 18.5 and 39.9 kg/m² (including normal weight, overweight, and obese individuals)
• Own a smartphone

Exclusion Criteria:

• Having a multiple pregnancy
• Pre-pregnancy/starting BMI <18.5 kg/m² (underweight) or BMI >40 kg/m² (morbidly obese) (due to the need for specialized nutrition/exercise counseling)
• Pregnant following fertility treatment
• Having a diagnosis of diabetes prior to pregnancy
• Having a diagnosis of or a history of eating and nutrition disorders
• Having a history of bariatric surgery
• Having diagnosed thyroid disease and/or adrenal disease
• Having a psychiatric illness or history of psychiatric illness
• Having a physical disability
• Pregnant women who do not wish to participate in the study

The planned study will be a single-blind trial. Due to the nature of the study, blinding of the researcher will not be possible, but participant blinding and statistician blinding will be implemented. During the data analysis phase, the statistical unit will perform the analyses without knowing which data belong to the intervention group and which belong to the control group. This will ensure the blinding of the statistician.