[1] Oslo Univ Hosp, Dept Pediat, Oslo, Norway
[2] Asker & Baerum Hosp, Dept Microbiol, Baerum, Norway
[3] Univ Oslo, Akershus Univ Hosp, Dept Obstet & Gynecol, Oslo, Norway
[4] Univ Oslo, Akershus Univ Hosp, Med Fac Div, Oslo, Norway
[5] Norwegian Inst Publ Hlth, Div Epidemiol, Oslo, Norway