We conducted a randomized control trial to assess the differences in the increase in PA among pregnant women who were included in the online social media intervention (Facebook group), the combination of online social media intervention and MI (Facebook and MI group) and control group. The study was conducted from January 2018 to April 2019 at the Clinic for Gynecology and Obstetrics, Clinical Centre of Serbia and Institute of Social Medicine, Medical Faculty, University of Belgrade. The women were approached after the prenatal screening at the 12th week of gestation. The intervention lasted through the second trimester. The reassessment was done at the end of the second trimester. The final analysis included 134 women.

The study was designed as a randomized controlled clinical trial and was conducted from January of 2018 to April 2019, at the Clinic for Gynaecology and Obstetrics, Clinical Centre of Serbia, Belgrade and the Institute for Social Medicine, Medical Faculty, University of Belgrade. The study compared the physical activity in the second trimester of pregnancy of the pregnant women who were included in the intervention groups (Facebook group and Facebook and MI group) and pregnant women from the control group. The intervention lasted for three months (from 12th till 28th week of gestation).

Total of 227 women were approached after the double test screening at the end of the first trimester (12th week of gestation). All women without medical or obstetric contraindications for physical activity were included in the study.

Baseline questionnaire consisted of 85 questions divided into five sections: social characteristics (age, place of residence, marital status, education, employment status, self-rated health, self-perceived financial status), anthropometric characteristics (height, weight, waist circumference), pregnancy and lifestyle characteristics (planning a pregnancy, symptoms present during the first trimester, and diet), and physical activity (leisure-time physical activity in the period of three months before pregnancy and physical activity during the first trimester).

At the baseline level, participants filled in the questionnaire and anthropometric measures were recorded. Anthropometric measures, height, weight and waist circumference, were measured by one of the researchers. Randomization was done after the initial baseline measurements. The follow-up measurement was done three months after the baseline, and all participants were asked to fill in the questionnaire which referred to physical activity in the second trimester.

The intervention involved social media intervention and Motivational interviewing.

Facebook group was specially designed for purposes of intervention and was administered by the research team. It was used to motivate the participants to be more physically active. The intervention used a hybrid approach. Motivation was done through health promotion campaign with motivational messages and pictures, journal articles, questions with the aim to spark a discussion on barriers or possibilities for physical activity ('What is your favourite form of physical activity?', 'Do you think that physical activity is safe during pregnancy?', 'Would you like to be more active during pregnancy?', etc). Facebook discussion group used a participatory approach and all members were allowed to post motivational messages or questions for their peers. The reports and photos from organized events (walking, yoga) were also posted regularly, so all participants could follow the level of participation of others. The research team shared information on free sports activities for pregnant women in multiple sports centres in Belgrade, as well as, specially organized events (walking around the lake, yoga).

Women who were randomized in the MI group went through two sessions of MI with the principal researcher. The first MI session was scheduled in the week after the inclusion in the study and the second was after 10-14 days. Each session lasted between 30 and 45 minutes. The principal researcher, who conducted the MI went to certified international training in MI.

Inclusion Criteria:

• Women without medical or obstetric contraindication for physical activity during pregnancy.

Exclusion Criteria:

• known medical or obstetric contraindication for physical activity during pregnancy;
• heavy smoking (>20 cigarettes per day)
• mental inability to understand and fill in the questionnaire.