1. Department of Obstetrics and Gynecology; Sørlandet Hospital; Kristiansand Norway
2. Department of Research; Sørlandet Hospital; Kristiansand Norway
3. Department of Public Health, Sports and Nutrition; University of Agder; Kristiansand Norway
4. Oslo Centre of Biostatistics and Epidemiology; Research Support Services; Oslo University Hospital; Oslo Norway
5. Section of Obstetrics, Women and Children's Division; Oslo University Hospital; University of Oslo; Oslo Norway