1 Edinburgh Medical School, University of Edinburgh, 49 Little France Crescent, Edinburgh, EH16 4SA, UK.
2 Department of Clinical Psychology, NHS Lothian, Astley Ainslie Hospital, 133 Grange Loan, Edinburgh, EH9 2HL, UK.
3 Nuffield Department of Women's and Reproductive Health, University of Oxford, Level 3, The Women's Centre, John Radcliffe Hospital, Headley Way, Oxford, OX3 9DU, UK.
4 MRC Centre for Reproductive Health, University of Edinburgh, Queen's Medical Research Institute, 47 Little France Crescent, Edinburgh, EH16 4TJ, UK. Electronic address: andrew.horne@ed.ac.uk.