1. Departments of Epidemiology and
2. Departments of Obstetrics and Gynecology and
3. Biostatistics, School of Public Health, Aarhus University, Aarhus, Denmark;
4. Orthopedic Surgery, Aarhus University Hospital, Aarhus, Denmark; and
5. Department of Epidemiology, School of Public Health, University of California, Los Angeles, California