Brain development starts in utero, and the fetal brain can already be affected by the environment, including chemical exposures and maternal health characteristics. These factors range from exposures to large quantities of teratogens (such as alcohol) to variations in the behaviors and characteristics of healthy individuals (such as age, diet, and subclinical levels of depressive and anxiety symptoms), which can nonetheless have long-lasting adverse effects.

In this thesis, we reviewed the literature on the effects of prenatal exposures on human neurodevelopment, as well as cognitive, behavioral, and health outcomes. In Study I we found that prenatal exposures are often reported poorly in infant neuroimaging studies and gave recommendations for reporting in future studies.

In Study II, we examined which early life factors predicted cortical structure in 5-year-olds. The results from Study II were utilized to make an informed decision regarding confounders in future studies in the 5-year-old neuroimaging sample of the FinnBrain Birth Cohort study. In Study III, we explored the cortical structural correlates of non-verbal ability in 5-year-olds. The findings were generally in line with prior results from adult and adolescent studies, with the important addition of a positive association between gray matter volume and surface area in the right medial occipital region and non-verbal ability as well as visual abstract reasoning ability.

Finally, in Study IV, we compared the results from two common segmentation tools, FSL-FIRST and FreeSurfer, against manual segmentation in the hippocampus and subcortical structures. Overall, the agreement with manual segmentation was good, although results were suboptimal for the hippocampus, amygdala, and nucleus accumbens, and careful visual quality control is still recommended.

This thesis summarized different perinatal factors affecting the developing brain, and ensured the high quality of our neuroimaging data. This foundational work, together with the multidisciplinary, longitudinal data collection in the FinnBrain Birth Cohort study, can be used to discover how environmental factors affect brain development.