[1] Haukeland Hosp, Dept Heart Dis, N-5021 Bergen, Norway
[2] Univ Bergen, Inst Med, Bergen, Norway
[3] Univ Bergen, Dept Publ Hlth & Primary Hlth Care, Bergen, Norway
[4] Norwegian Inst Publ Hlth, Med Birth Registry Norway, Bergen, Norway
[5] Univ Hosp No Norway, Tromso, Norway