[1] Massachusetts Gen Hosp, Crohns & Colitis Ctr, Boston, MA 02114 USA
[2] Harvard Med Sch, Boston, MA 02115 USA
[3] Karolinska Inst, Div Clin Epidemiol, Dept Med Solna, Stockholm, Sweden
[4] Karolinska Inst, Dept Med Epidemiol & Biostat, Stockholm, Sweden
[5] Stockholm South Gen Hosp, Sachs Children & Youth Hosp, Stockholm, Sweden
[6] Orebro Univ Hosp, Dept Pediat, Orebro, Sweden
[7] Univ Nottingham, Div Epidemiol & Publ Hlth, Sch Med, Nottingham, England
[8] Columbia Univ Coll Phys & Surg, Dept Med, New York, NY USA