1. Department of Obstetrics and Department of Gynaecology, Oslo University Hospital, Ullevål, Pb 4965, Nydalen, 0424 Oslo, Norway
2. Faculty of Medicine, University of Oslo, Norway
3. Women's Health and Perinatology Research Group, Department of Clinical Medicine, University of Tromsø, 9019 Tromsø, Norway
4. Unit of Biostatistics and Epidemiology, Oslo University Hospital, Pb 4965, Nydalen, 0424 Oslo, Norway