1. Department of Epidemiology, Graduate School of Public Health; University of Pittsburgh; Pittsburgh Pennsylvania USA
2. Department of Obstetrics, Gynecology, and Reproductive Sciences, School of Medicine; University of Pittsburgh; Pittsburgh Pennsylvania USA
3. Magee-Womens Research Institute; Pittsburgh Pennsylvania USA
4. Department of Obstetrics and Gynaecology; University of British Columbia; Vancouver British Columbia Canada
5. Department of Epidemiology, Rollins School of Public Health; Emory University; Atlanta Georgia USA
6. Division of Epidemiology; University of California at Berkeley School of Public Health; Berkeley California USA