The xSmoker app was developed with European Commission funding. xSmoker is a digital health coaching mobile app that helps individuals stop smoking and remain smoke free. The initial version of the xSmoker application was received from the developers and translated into Romanian by the research team. The content has been divided into three main sections: "Daily Tips" (approximately 630 items), which includes information on the beneficial effects of quitting smoking, "Panic Tips" (approximately 100 items), which can be accessed at that time when risk of smoking relapse is high, as well as a section called "Library" (about 120 items), where detailed information on the topics included in the first two sections is provided.

The xSmoker app + phone text messages with content based on the Motivation and Problem Solving approach and informed by our prior work. The investigators developed six categories of messages sent to participants: (1)Importance and trust, (2)Fear of relapse, (3)Partner support, (4)Breastfeeding, (5)The need to smoke, and (6)Relapse. Four major objectives were established based on the content of the SMS text messages, with the help of the literature: (1)supporting motivation, (2)supporting self-efficacy, (3)supporting dyadic effectiveness, and (4)developing problem-solving skills. The messages were delivered using Textit, a platform for visually building interactive SMS applications (htpps://textit.in). All the messages were uploaded in the platform and different flows and sub-flows were created for every day of the intervention to automatize the process of SMS delivery. A combination of trigger words and skip patterns was used in order to tailor the messages.

Usual postnatal care

xSmoker - mHealth intervention consisted of using the app xSmoker

PPA, participants who reported 'no smoking, not even a puff' in the previous 7 days are mailed a saliva cotinine test and asked to send at least one photo of the completed test. Those with a salivary cotinine <10 ng/mL, were considered confirmed nonsmokers.

PA was defined based on the question "Have you smoked tobacco cigarettes over the past 6 months", with PA failure determined by the answers "Yes, I smoked, but never a whole cigarette," "Yes, I smoked cigarettes, but only occasionally," or "Yes, I smoked cigarettes daily for a period (or periods) of time."