1. Department of Obstetrics and Gynaecology; University College London Hospitals NHS Foundation Trust; EGA Wing; 235 Euston Road; London; NW1 2BU; UK
2. Early Pregnancy and Gynaecology Diagnostic Unit; University College London Hospitals NHS Foundation Trust; EGA Wing; 235 Euston Road; London; NW1 2BU; UK
3. Department of Obstetrics and Gynaecology; North Middlesex University Hospital NHS Trust; Sterling Way; London; N18 1QX; UK