1. Department of Gynaecology and Obstetrics, West China Second Hospital, Sichuan University, Chengdu, PR China;
2. Key Laboratory of Birth Defects and Related Diseases of Women and Children, Ministry of Education, Sichuan University, Chengdu, PR China;
3. International Centre for Reproductive Health, Ghent University, Ghent, Belgium;
4. School of Public Health, Université Libre de Bruxelles, Brussels, Belgium;
5. Department of Obstetrics and Gynaecology, Aga Khan University, Nairobi, Kenya