Ystrom, Eivind, Ted Reichborn-Kjennerud, Kristian Tambs, Per Magnus, Anne Mari Torgersen, and Kristin Gustavson. “Multiple Births and Maternal Mental Health from Pregnancy to 5 Years After Birth: A Longitudinal Population-Based Cohort Study”. Norsk Epidemiologi 24, no. 1-2 (December 29, 2014). Accessed August 15, 2020. https://www.ntnu.no/ojs/index.php/norepid/article/view/1823.