Author: Kristin Samuelsen. Title: Emerging Genetic and Environmental Risk Factors for Prenatal and Postnatal Depression. Supervisors: Main supervisor Eivind Ystrøm, and co-supervisors Line C. Gjerde and Espen M. Eilertsen. Background: Many women experience depression both during pregnancy (prenatal) and following delivery (postnatal). Although there is some literature on risk factors, there is a lack of studies investigating heritability. Thus, it is largely unknown to what extent genetic and environmental factors contribute to depression at these time points, as well as whether they are similar across timing. Research on these questions may yield important insights into whether depression prior to and following childbirth is the same, similar or distinct constructs. Research aims: 1) Estimate the relative importance of genetic and environmental factors for depression during and after pregnancy, and 2) Estimate the genetic and environmental correlation between depression during and after pregnancy, i.e. estimate the extent of genetic innovation. Sample: The sample used in this study is a subsample of the prospective, ongoing pregnancy cohort study Norwegian Mother and Child Cohort Study (MoBa), conducted by the Norwegian Institute of Public Health (N = 64 monozygotic (MZ) twin pairs, 35 dizygotic (DZ) twin pairs, 5540 full sibling (FS) pairs, and 400 half siblings (HS) pairs. Research design: A quantitative, extended twin study design, including siblings was applied. Methods: Measurement was conducted at week 30 of pregnancy and 6 months following delivery, using SCL-8, an abbreviated version of SCL-90. Univariate and bivariate twin modeling was conducted. Results: The relative importance of genes and environment for prenatal depressive symptoms was estimated at 16.2%, and 83.8%, respectively, and at 25.7% and 74.3% postnatally. Estimated correlation between pre and postnatal depressive symptoms was at 1.00 for genetic effects, and .36 for environmental effects. The unstandardized genetic effects for postnatal depressive symptoms were 172% of that of prenatal depressive symptoms, showing a quantitative, but a lack of a qualitative, gene-environment interaction (GxE). That is, the same genetic factors had a stronger impact postnatally. Conclusions: The findings indicate that the most important risk factors for pre and postnatal depression are environmental in their nature, yet they appear to be different dependent on the timing. The same genetic factors appear to influence depression at both time points, but to a stronger degree postnatally. This implicates that prevention efforts to depression occurring at this time should be predominantly aimed at reducing environmental stressors of depression specific to each timing.