1 Fetal Medicine Unit, Department of Obstetrics and Gynaecology, St George's University Hospitals NHS Foundation Trust, St George's University of London, London SW17 0QT, UK.
2 Middle East Technical University, Faculty of Arts and Sciences, Department of Statistics, Ankara, Turkey; Koc University, School of Medicine, Department of Obstetrics and Gynaecology, Istanbul, Turkey.
3 Department of Health Service Research and Policy, London School of Hygiene and Tropical Medicine, London, UK.
4 The Royal College of Obstetricians and Gynaecologists, London, UK; University College London Hospitals NHS Foundation Trust, London, UK.
5 Norfolk and Norwich University Hospitals NHS Foundation Trust, Norwich, Norfolk, UK.
6 The Royal College of Obstetricians and Gynaecologists, London, UK; North Bristol NHS Trust Department of Women's Health, Westbury on Trym, UK.
7 Institute of Metabolism and Systems Research, WHO Collaborating Centre for Women's Health, University of Birmingham, Birmingham, UK.
8 Paediatric Infectious Diseases Research Group and Vaccine Institute, Institute of Infection and Immunity, St George's University of London, London SW17 0QT, UK.
9 Paediatric Infectious Diseases Research Group and Vaccine Institute, Institute of Infection and Immunity, St George's University of London, London SW17 0QT, UK; Immunisation and Countermeasures Division, Public Health England, London, UK; British Paediatric Surveillance Unit, Royal College of Paediatrics and Child Health, London, UK.
10 Department of Women and Children's Health, School of Life Course Sciences, King's College London, London, UK.
11 British Paediatric Surveillance Unit, Royal College of Paediatrics and Child Health, London, UK.
12 Fetal Medicine Unit, Department of Obstetrics and Gynaecology, St George's University Hospitals NHS Foundation Trust, 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 SW17 0QT, UK. Electronic address: akhalil@sgul.ac.uk.