Academic Centre for Women's Health, North Bristol NHS Trust, Bristol, United Kingdom; School of Clinical Sciences, University of Bristol, 69 St Michael's Hill, Bristol, BS2 8DZ, United Kingdom; School of Clinical Sciences, University of Bristol, 69 St Michael's Hill, Bristol, BS2 8DZ, United Kingdom; BresMed, 84 Queen Street, Sheffield, S1 2DW, United Kingdom; Department of Statistical Science, University College London, Gower Street, London, WC1E 6BT, United Kingdom.