1. Centre for Research into Circulating Fetal Nucleic Acids, Li Ka Shing Institute of Health Sciences, Hong Kong, China;
2. Department of Chemical Pathology, The Chinese University of Hong Kong, Hong Kong, China; and
3. Department of Obstetrics and Gynaecology and
4. Katharine Dormandy Haemophilia and Thrombosis Center, Royal Free Hospital, London, United Kingdom