1. Institute for Social Medicine and Epidemiology, University of Luebeck, 23562 Luebeck, Germany
2. Department of Obstetrics and Gynaecology, Ernst von Bergmann Clinic, 14467 Potsdam, Germany
3. Department of Obstetrics and Gynaecology, University Hospital of Schleswig-Holstein, Campus Luebeck, 23562 Luebeck, Germany