Mobile health (mhealth) refers to delivering health care and health promotion through mobile devices \[1\]. Advances of mobile technologies have led to increased use of mhealth interventions for effective healthy infant feeding practices and breastfeeding promotion. A meta-analysis reviewed that mhealth significantly improved exclusive breastfeeding initiation, breastfeeding attitude and knowledge \[2\]. Whether more interactive and adaptive mHealth platforms, including smartphone apps and social networking tools, could further improve breastfeeding outcomes remains understudied. Smartphones have become new channels and tools for information acquisition and exchange and have been well adopted by the general public. In 2019, the number of smartphone users in Hong Kong was estimated to reach 6.5 million. By the year 2025, it is predicted that 93% of the population in Hong Kong will use a smartphone \[3\]. Among instant messaging apps for smartphones, WhatsApp is regarded as one of the largest social network platforms used in Hong Kong. WhatsApp is a free all in one communications app widely used for sending text and voice messages, video calls. Online discussion platforms have been used, acceptable, affordable and accessible in previous years for promoting physical activity \[4\], a healthy diet \[5\], smoking cessation \[6\] and reducing alcohol use \[7\]. Interventions using these communication tools tend to have high uptake due to personalized intervention and instant response without having time and location limitations. Whether WhatsApp can be used as a communication tool to positively change selected behaviours has yet to be assessed its effects on breastfeeding outcomes.

With the current pandemic and social distancing measures, this study offers insight on the practicality and feasibility of supporting Chinese mothers who are housebound due to Chinese traditional cultural "doing the month" in which Chinese mothers are housebound during the first month post-birth. In addition, due to the pandemic, there is limited face to face antenatal and postnatal support from the health services. Unresolved problems and inadequate support with infant feeding may result in a decision to stop exclusively breastfeeding. Our previous qualitative study suggested that WhatsApp messaging might be a viable way of promoting breastfeeding (unpublished data). Therefore, a pilot study is developed to address an important service gap in Hong Kong to promote and sustain exclusive breastfeeding. However, there might be potential barriers to successful implementation and to evaluating the effectiveness of the innovation intervention. This is the first WhatsApp breastfeeding support programme provided by peer supporters in Hong Kong. In Hong Kong, the most recent breastfeeding survey showed that the exclusive breastfeeding rate at 6 months was 26.3% in Hong Kong \[8\]. Furthermore, more than 88% of mothers initiated breastfeeding but half of breastfeeding mothers never exclusively breastfed their babies \[8\]. Despite the importance of exclusive breastfeeding for the first 6 months postpartum, the maintenance of exclusive breastfeeding is a major public health issue particularly in Hong Kong where the rate on exclusive breastfeeding duration indicated that it tailed off during the first 2 months \[8\]. This indicates novel approaches are needed to engage hard to reach mothers who encounter breastfeeding problems or support during the first 6 months.

Existing services mainly target mothers in clinical or primary care settings delivered by health professionals for promoting breastfeeding. Hong Kong has an extensive smartphone penetration rate (78% in 2020). Therefore, before proceeding with a full scale randomized controlled trial (RCT), a feasibility and pilot study is needed to identify if a WhatsApp online group on breastfeeding by peer counsellors, delivered through instant/video messaging support, designed to improve breastfeeding outcomes is feasible and acceptable. The main objective of the study is to determine the feasibility of conducting a WhatsApp online group on breastfeeding by peer counsellors over a six-month period in postpartum Chinese women in Hong Kong.