1. Institute for Molecular Medicine Finland University of Helsinki Helsinki Finland
2. Department of Epidemiology Biostatistics and Biodemography, University of Southern Denmark Odense Denmark
3. The Danish Twin Registry University of Southern Denmark Odense Denmark
4. Center for Fertility and Health The Norwegian Institute of Public Health Oslo Norway
5. Department of Medical Epidemiology and Biostatistics Karolinska Institutet Stockholm Sweden
6. Clinical Effectiveness Group Institute of Health, University of Oslo Oslo Norway