[1] Norwegian Inst Publ Hlth, Div Epidemiol, Box 4404 Nydalen, N-0403 Oslo, Norway
[2] Karolinska Inst, Dept Med Epidemiol & Biostat, Stockholm, Sweden
[3] Ostfold Hosp Trust, Dept Pediat, Fredrikstad, Norway
[4] Norra Alvsborg Hosp, Dept Obstet & Gynecol, Trollhattan, Sweden
[5] Orebro Univ Hosp, Dept Pediat, Orebro, Sweden