[1] Sorlandet Hosp HF, Dept Obstet & Gynaecol, Res Unit, Kristiansand, Arendal, Norway
[2] Univ Oslo, Inst Clin Med, Fac Med, Oslo, Norway
[3] Norwegian Univ Sci & Technol, HUNT Ctr Mol & Clin Epidemiol, Dept Publ Hlth & Nursing, Trondheim, Norway
[4] Trondheim Reg & Univ Hosp, St Olavs Hosp, Clin Med, Dept Endocrinol, Trondheim, Norway
[5] Oslo Univ Hosp, Oslo Ctr Biostat & Epidemiol Res Support Serv, Oslo, Norway
[6] Oslo Metropolitan Univ, Fac Hlth Sci, Oslo, Norway
[7] Oslo Univ Hosp, Dept Obstet, Div Obstet & Gynaecol, Oslo, Norway
[8] Sorlandet Sykehus HF, Postboks 416 Lundsiden, N-4604 Kristiansand, Norway