1. University of Bern; Division of Clinical Epidemiology and Biostatistics, Institute of Social and Preventive Medicine; Finkenhubelweg 11 Bern Switzerland CH-3012
2. Leiden University Medical Center; Department of Gynaecology, Division of Reproductive Medicine; PO Box 9600 Albinusdreef 2 Leiden Netherlands NL 2300 RC
3. World Health Organization; Department of Reproductive Health and Research; 20 Rue Appia Geneva 27 Switzerland CH-1211