[1] City Univ London, Ctr Maternal & Child Hlth Res, Sch Hlth & Psychol Sci, Northampton Sq, London EC1V 0HB, England
[2] Univ Stirling, Nursing Midwifery & Allied Hlth Profess Res Unit, Pathfoot Bldg, Stirling FK9 4LA, Scotland
[3] Oxford Populat Hlth, Natl Perinatal Epidemiol Unit, Old Rd Campus, Oxford OX3 7LF, England
[4] Southern Cross Univ, Gold Coast Airport, Terminal Dr, Bilinga, Qld 4225, Australia