1 Fetal Medicine Unit, St George's University Hospitals NHS Foundation Trust, University of London, London, UK.
2 Vascular Biology Research Centre, Molecular and Clinical Sciences Research Institute, St George's University of London, London, UK.
3 Department of Women's and Children's Health, Karolinska Institute, Stockholm, Sweden.
4 Astrid Lindgren Children's Hospital, Karolinska University Hospital, Stockholm, Sweden.
5 The Royal College of Obstetricians and Gynaecologists, London, UK.
6 University College London Hospitals NHS Foundation Trust, London, UK.
7 Norfolk and Norwich University Hospitals NHS Foundation Trust, Norwich, Norfolk, UK.
8 North Bristol NHS Trust Department of Women's Health, Westbury on Trym, UK.
9 Imperial College London, London, UK.
10 Paediatric Infectious Diseases Research Group and Vaccine Institute, Institute of Infection and Immunity, St George's University of London, London, UK.
11 Immunisation and Countermeasures Division, Public Health, England.
12 British Paediatric Surveillance Unit, Royal College of Paediatrics and Child Health, London, UK.