[1] Karolinska Univ Hosp, Astrid Lindgren Childrens Hosp Danderyd, Dept Neonatol, Stockholm, Sweden
[2] Uppsala Univ, Uppsala Clin Res Ctr, Dept Med Sci, Uppsala, Sweden
[3] Karolinska Inst, Dept Med Epidemiol & Biostat, S-17177 Stockholm, Sweden
[4] Orebro Univ Hosp, Dept Paediat, Orebro, Sweden