1. Unit of Paediatric and Perinatal Epidemiology (I.R., P.E.), Department of Community-Based Medicine, University of Bristol, Bristol BS8 1TQ, United Kingdom
2. Department of Social Medicine (C.M., D.G.), University of Bristol, Bristol BS8 2PR, United Kingdom
3. Department of Paediatrics (D.D.), University of Cambridge, Addenbrookes Hospital, Cambridge CB2 2QQ, United Kingdom
4. Division of Surgery (J.H.), University of Bristol, Bristol, BS8 2HW, United Kingdom