1. Department of Obstetrics and Gynecology, Beijing LuHe hospital, Capital Medical University, Beijing, China
2. Clinical Epidemiology Program, Ottawa Health Research institute, Ottawa, Canada
3. Department of Obstetrics and Gynecology, University of Ottawa, Ottawa, Canada
4. School of Epidemiology and Public Health, University of Ottawa, Ottawa, Canada
5. Department of Nutrition, Beijing LuHe hospital, Capital Medical University, Beijing, China
6. Department of Obstetrics and Gynecology, Queen’s University, Kingston, Canada.