1. Department of Gynaecology and Obstetrics; Haukeland University Hospital; Bergen Norway
2. Department of Global Public Health and Primary Care; University of Bergen; Bergen Norway
3. Medical Birth Registry of Norway; Norwegian Institute of Public Health; Bergen Norway
4. Department of Obstetrics and Gynaecology; Fertility Clinic; St Olav's University Hospital; Trondheim Norway
5. Department of Public Health; Norwegian University of Science and Technology; Trondheim Norway