Health and Development. It was part of the MoBaKinder-project supported by the Norwegian Research Council through the BEDREHELSE programme. I would like to thank many people for contributing to my completion of this project.First, I thank all the participants in MoBa for completing all the questionnaires. There would not be a project without you. I would also like to thank University of Oslo for the opportunity to submit this thesis.My deepest gratitude goes to my supervisor Mari Vaage Wang and co-supervisor Espen Røysamb. Mari, you have been the rock in this project, and everything from a supervisor to a friend. I have learnt so much from you! Thank you for always staying positive, yet realistic; for being fun and casual, yet "green" enough to get things done; and more than anything for your ability to motivate and encourage me even when life happens and challenges the original plans. Espen, I want to thank you for your open-minded views, beneficial comments, and engagement. I highly appreciate your pedagogical ways of discussing topics and suggestions. I want to thank the whole MoBaKinder crew: Mari, Ragnhild, Nina, Silje, Erik, and Elise. You have been a great team for help, guidance and discussions, and for hiking, skiing, dining and laughing, and always leaving the door open. Thank you Silje, for your "sprintcalendar" counting down the final days with chocolate and sweets. To the wider project group: thank you for your great interest and engagement in project-meetings, and in particular Ratib and Thomas for co-authorships on papers. Your engagement has been encouraging and highly valuable.To the department director Heidi Aase and all my colleagues at NIPH, Department of Child Health and Development. Thank you for making a great atmosphere at work, especially around the lunch table and at "waffle and workout"-Fridays.To my fellow PhD-students at NIPH, especially Thea, Maria, Anne Kristine, Elise, Nina, and Linn. Thank you for travelling this journey together. I believe a team of peers like you to share this experience with have been essential in these strange years. I am lucky to have spent these years with you at the office, in seminars, on screen, at lunch, drinks, walks, ice cream breaks and joy. Special thanks to my co-inhabitants of the corner office: Anne II Kristine and Maria. Thank you for being great colleagues and wonderful friends both at NIPH and outside.My sincere gratitude to my brilliant family, particularly Rolf, Herdis and Karina.Thank you for giving me a safe and warm home and childhood; for always being cheerful, supportive and caring; and for being favourite catering, travel agency and companions, car rental, and entertainment crew.Thank you to all my wonderful friends, including Gjeng, for being superb friends, giving me plenty of opportunities to reload, enjoy new travels, experiences, sports, feasts and fun. You make life great! Thank you for also engaging in the topic of my work. Additional thanks to Jess for proof reading. This journey has, as most journeys, not been a walk in the par...