Sample size estimation
We conducted a pilot study to assist with the sample size estimation. Twenty pregnant women who volunteered to participate were randomized to complete the questionnaire before the pretest counseling. Their mean knowledge score was 7.1 ± 4.6 (Full scores: 18). After traditional counseling, the mean score increased to 9.7 ± 4.6, which was about 36% increase. We predicted that watching the video may lead to double incremental knowledge score (70% increase) at 12.1 ± 4.6. Assuming a two-sided alpha of 0.05 and 90% power to detect a significant difference, this meant we needed 80 participants per group. We estimated the potential for 10% data loss and therefore aimed to enroll 88 participants in each group to give a total of 176 participants.