1. Women and Children's Division; Norwegian Resource Centre for Women's Health; Oslo University Hospital; Rikshospitalet Norway
2. Medical Birth Registry of Norway; Norwegian Institute of Public Health; Bergen Norway
3. Department of Global Public Health and Primary Care; University of Bergen; Bergen Norway
4. University of Oslo; Oslo Norway
5. Department of Gynaecology; Oslo University Hospital Rikshospitalet; Oslo Norway
6. Division of Epidemiology; Norwegian Institute of Public Health; Oslo Norway
7. Department of Obstetrics; Oslo University Hospital Rikshospitalet; Oslo Norway