[1] Sandwell & W Birmingham NHS Trust, Univ Dept Med, Haemostasis Thrombosis & Vasc Biol Unit, Birmingham B18 7QH, W Midlands, England
[2] Sandwell & W Birmingham NHS Trust, Dept Surg, Birmingham B18 7QH, W Midlands, England
[3] John Radcliffe Hosp, Nuffield Dept Obstet & Gynaecol, Oxford OX3 9DU, England