1. Department of Genito-Urinary Medicine, Northwick Park and St Mark's NHS Trust, Harrow, Middlesex HA1 3AU
2. University College London Medical School, Gower Street, London WC1E 6BT
3. Department of Obstetrics and Gynaecology, Northwick Park and St Mark's NHS Trust, Harrow, Middlesex HA1 3UJ
4. MRC Sexually Transmitted Diseases Research Group, St Mary's Hospital, London W2 1NY, UK