The 3D-Transition study is a follow-up of the 3D Cohort pregnancy study (NCT03113331, which covered from the 1st trimester of pregnancy to age 2 years) as the children transition into kindergarten and first grade. It aims at clarifying prenatal and preschool predictors of challenging and successful transitions to school as measured by mental health and academic outcomes.

Broad goal: This study seeks to understand how early vulnerabilities (externalizing or internalizing symptoms, or low neurodevelopmental/academic skills) turn into a rate of 10-15% of externalizing and internalizing mental health problems in school-aged children.

Rationale: Several lines of research guide this study. Functional impairments emerge for some children challenged by the transition to kindergarten and grade school. This risk is also thought to be programmed very early in life through cumulative early perinatal and psychosocial adversity. Further, interactions between the child, family and school environment during the transition may mitigate future impairments and this may be different for boys and girls. Finally, stress regulation mechanisms during the transition seem linked to early child vulnerabilities. This design allows us to contrast any combination of these mechanisms.

Aims: This study will clarify the role of:

1. a mismatch between some child preschool vulnerability (externalizing, or internalizing symptoms, or low neurodevelopmental/academic skills) and the challenges of the transition from preschool to formal schooling;
2. the experience of perinatal adversity and home chaos during child first years;
3. supportive environments (high quality families or schools) during the school transition, and child sex; and
4. the child stress response to the school transition, as assessed through salivary cortisol.

Method: Building on an existing pregnancy cohort (NCT03113331, which was structured around a triadic (mother-partner-child) framework, and which covered from the 1st trimester of pregnancy to age 2 years), the investigators have followed at least once 939 of 1551 families that agreed to a further follow-up past the initial study. Children in this cohort, who were seen in 3 waves based on age on September 30th, entered kindergarten in the fall of 2016, 2017, and 2018, respectively. A cohort-sequential longitudinal research design spanning pre-kindergarten to 2nd grade (ages 4 to 8 years), was implemented to follow each wave 6 times over 4 years. Cascade models will be used to address aims 1-3. The stress hormone cortisol will be examined on 11 days spread over a 16 months period for a subsample of 384 children to address aim 4, using growth curves models. This design is well suited and sufficiently powered to examine change processes over time, controlling for potential differences in waves and time of measurement effects.

Additional data: The Research Ethics Committee authorized two Corona Virus (COVID-19) supplemental data collections without additional consent for the spring of 2020 and the spring of 2021. However, cohorts 1 and 2, which had completed participation in 2019 and 2020, were re-consented, and this extended the maximum age range to 10 and 9 years, respectively. As of 2022-05-03 investigators are in the process of requesting approval for an additional COVID-19 follow-up.

Expected outcomes: Public health interventions are being put in place to help children manage the transition to school on the basis of little evidence. This longitudinal research will provide a greater understanding of individual and environmental factors linked to children's adaptation during the perinatal period and transitions to school. As such, mental health prevention research will be better informed on the developmental timing of individual and environmental targets that need to be considered in a developmental framework.

Inclusion Criteria:

• All participating families from Clinical Trials NCT03113331 who had accepted a follow-up past the original pregnancy to age 2-years time span of that protocol.

Exclusion Criteria:

• None for this follow up. See Clinical Trials NCT03113331 for original exclusion criteria.

General population sample from the main university hospitals affiliated with the province of Québec, Canada's 4 faculties of medicine between November 2010 and March 2013, included.

The procedures by which de-identified data will be accessible are being clarified as rules and regulations are currently under study in this jurisdiction. In the meantime the investigators can be contacted for updates - see "More information"

Partial data from earlier waves may be available upon request. The bulk of the data should be available by March 2023.

Data are under the jurisdiction of the Centre Hospitalier Universitaire (CHU) Ste-Justine Research Ethics Committee and subject to current provincial and national privacy laws guiding their ethical use in Québec, Canada. To submit a request, please visit the 3D-Cohort Study website or contact the principal investigator.