The popularity of mobile period tracker apps, designed to help women track their periods and fertility, has skyrocketed over the past decade. The target audience is people who menstruate, most often women. There are numerous articles on personalisation in mobile apps, but personalisation in mobile period tracker apps has been little studied. Therefore, in this thesis, I analyse the ways of collecting information about users and personalising the user's account in the period tracker apps, as well as whether this personalisation meets the needs of female users. My research question is: How could the personalisation of mobile period tracker apps be improved through design to meet real users' needs? The study builds on the user experience (UX) design process and consists of the following phases: UX research, design, and user testing. In addition, it includes an ideation part. I used post-structural feminist theory from the perspective of Judith Butler's work as a framework. I conducted seven semi-structured interviews with female users who actively use period tracker apps. I studied three chosen period tracker apps using the walkthrough method and conducted a co-design workshop using the themes identified through the thematic analysis of the interviews. The research output is a design solution tested on five participants of the study and then iterated. Through my work, I contribute to studies of reproductive technologies and the field of feminist human-computer interaction (HCI) through suggestions on personalisation.