The participants will receive a wrist-worn activity tracker (Garmin VívoSmart 5) including clear instructions on how to use the tracker and how to connect to a software (Fitrockr). We ask the participants to wear the tracker 24/7 during the period they are trying to conceive and during the first trimester of their pregnancy if they conceive within 12 months to measure physical activity. Participants in the control group will not receive any PA feedback.

Participants are advised to maintain their usual activity habits.

The participants will receive a wrist-worn activity tracker (Garmin VívoSmart 5) including clear instructions on how to use it. They will be asked to wear the tracker 24/7 during the period they are trying to conceive plus the first trimester of their pregnancy. They will receive information material on the Danish national guidelines on PA, health benefits from PA, and suggestions on how to reduce sedentary behavior and increase daily activity. Based on the current PA level, they are asked to set personal goals to meet the PA recommendations and to split up sedentary behavior. They will receive a phone call two and five weeks after they received the activity tracker, to stimulate the participant's motivation to be physically active, to set goals for maintaining or increasing PA and to facilitate a revision of the goals if needed.

The conversations will be inspired by motivational interviewing and the principles of this motivational technique.

All participants will receive a wrist-worn activity tracker (Garmin VívoSmart 5) which will be connected with an application, Fitrockr. Data from the devices will be linked to data from the Snartforældre-questionnaires. The participants are asked to wear the tracker 24/7 during preconception and during the first trimester of their pregnancy if they conceive within 12 months.

Participants will receive information material and be asked to set goals for their PA level. To follow-up on the goal setting, they will receive two phone calls, where the conversations will be inspired by motivational interviewing.

Physical activity will be objectively measured continuously (24/7) using wrist-worn activity trackers with triaxial accelerometry.

TTP will be estimated by using variables from baseline and follow-up questionnaires. In each follow-up questionnaire, women report the date of the first day of their last menstrual period (LMP), whether they are currently pregnant, and whether they experienced any other pregnancies (SAB, induced abortion, or ectopic pregnancy) since their last questionnaire. For irregularly cycling women, we estimate cycle length based on baseline LMP date, expected date of next menses, and LMP recorded during follow-up.

Spontaneous abortion (SAB), defined as an intrauterine pregnancy loss before week 22 of gestation, will be identified from both bimonthly follow-up questionnaires and registries. In follow-up questionnaires, women report if they have had a pregnancy loss since the date of the most recent completed questionnaire as well as the date of loss and the gestational age at loss. Using the civil registration number (CPR), we will retrieve data from the Danish National Patient Registry (DNPR) and the Danish Medical Birth Registry (DMBR) to identify SABs and induced abortions occurring after the baseline enrollment date. For SABs identified from both registry and questionnaire data, we will use data from the DNPR (based on early ultrasound fetometry or LMP) to ascertain the time of SAB.

A mixed methods process evaluation using the UK Medical Research Council's process evaluation framework to assess implementation (reach, fidelity, and dose) and mechanisms of impact of the intervention. We will collect data through motivational conversations and activity trackers, which provide both qualitative and quantitative insights.

Through motivational conversations with participants, we will capture their experiences, challenges, and suggestions regarding the intervention. This qualitative feedback helps us understand context-specific issues.

Data from activity trackers will offer objective measures of participant behavior and engagement with the intervention, allowing us to identify trends, such as compliance and changes in activity levels, that may indicate whether the intervention is being followed as intended.