A R A Aiken1, C E M Aiken2, J Trussell1, K A Guthrie3. 1. Office of Population Research, Princeton University, Princeton, NJ, USA. 2. Department of Obstetrics and Gynaecology and NIHR Cambridge Comprehensive Biomedical Research Centre, University of Cambridge, Cambridge, UK. 3. Sexual and Reproductive Healthcare Partnership, Hull, UK.