1. Department of Gynaecology; Oslo University Hospital; Oslo Norway
2. Faculty of Medicine; University of Oslo; Norway
3. Department of Obstetrics and Gynaecology; Vestfold Hospital; Tønsberg Norway
4. Department of Obstetrics and Gynaecology; Østfold Hospital; Fredrikstad Norway
5. Unit of Biostatistics and Epidemiology; Oslo University Hospital; Oslo Norway
6. Department of Obstetrics and Gynaecology; Asker and Baerum Hospital; Baerum Norway