1 Fetal Medicine Unit, St George's Hospital, St George's University of London, London SW17 0RE, UK; Vascular Biology Research Centre, Molecular and Clinical Sciences Research Institute, St George's University of London, London SW17 0RE, UK. Electronic address: akhalil@sgul.ac.uk.
2 The Portland Hospital for Women and Children, London, UK; Great Ormond Street Hospital, London, UK.
3 Paediatric Infectious Diseases Research Group, St George's University of London, London SW17 0RE, UK; Immunisation and Countermeasures Division, Public Health England, London, UK.
4 The Portland Hospital for Women and Children, London, UK.
5 The Royal College of Obstetricians and Gynaecologists, London, UK; University College London Hospitals National Health Service Foundation Trust, London, UK.