Clinical Trials Unit, London School of Hygiene and Tropical Medicine, London, WC1E 7HT, UK. Clinical Trials Unit, London School of Hygiene and Tropical Medicine, London, WC1E 7HT, UK. Department of Obstetrics & Gynaecology National Institute of Maternal and Child Health, College of Medicine, University of Ibadan, Ibadan, Nigeria. Department of Chemical Pathology, College of Medicine, University of Ibadan, Ibadan, Nigeria. Department of Obstetrics & Gynaecology, College of Medicine, University of Ibadan, Ibadan, Nigeria. Department of Obstetrics & Gynaecology, College of Medicine, University of Ibadan, Ibadan, Nigeria. Clinical Trials Unit, London School of Hygiene and Tropical Medicine, London, WC1E 7HT, UK. Department of Obstetrics & Gynaecology, College of Medicine, University of Ibadan, Ibadan, Nigeria. Department of Haematology, College of Medicine, University of Ibadan, Ibadan, Nigeria. Department of Obstetrics & Gynaecology, College of Medicine, University of Ibadan, Ibadan, Nigeria. Department of Obstetrics & Gynaecology National Institute of Maternal and Child Health, College of Medicine, University of Ibadan, Ibadan, Nigeria. Department of Haematology, College of Medicine, University of Ibadan, Ibadan, Nigeria. Department of Obstetrics & Gynaecology National Institute of Maternal and Child Health, College of Medicine, University of Ibadan, Ibadan, Nigeria. Institute for Medical Information Sciences, Biometry and Epidemiology, Ludwig Maximilian University of Munich, Munich, Germany. University Hospital of Obstetrics Gynaecology "Koco Gliozheni", Tirana, Albania. University Hospital of Obstetrics Gynaecology "Koco Gliozheni", Tirana, Albania. Thrombosis & Haemophilia Centre, Guy's & St Thomas' Trust, London, UK. Clinical Trials Unit, London School of Hygiene and Tropical Medicine, London, WC1E 7HT, UK. Clinical Trials Unit, London School of Hygiene and Tropical Medicine, London, WC1E 7HT, UK.