1 Fetal Medicine Unit, St George's Hospital, St George's University of London, London SW17 0QT, UK; Vascular Biology Research Centre, Molecular and Clinical Sciences Research Institute, St George's University of London, London, UK. Electronic address: akhalil@sgul.ac.uk.
2 Department of Women's and Children's Health, Karolinska Institutet, Stockholm, Sweden; Astrid Lindgren Children's Hospital, Karolinska University Hospital, Stockholm, Sweden.
3 Paediatric Infectious Diseases Research Group and Vaccine Institute, Institute of Infection and Immunity, St George's University of London, London, UK; Immunisation and Countermeasures Division, Public Health England, London, UK; British Paediatric Surveillance Unit, Royal College of Paediatrics and Child Health, London, UK.
4 The Royal College of Obstetricians and Gynaecologists, London, UK; University College London Hospitals NHS Foundation Trust, London, UK.