Mortality rate ratios according to characteristics of study population.

aMRRs were adjusted for country, age, sex, calendar year period, birth outcomes (birth weight, the Apgar score at 5 minutes, preterm birth), and maternal variables (age, parity, education, and social status).bBirth weight available period: 1979–2008 in Denmark, 1973–2006 in Sweden, 1987–2007 in Finland; parity available period: 1968–2008 in Denmark, 1973–2006 in Sweden, 1987–2007 in Finland; Gestational age and singleton available period: 1973–2008 in Denmark, 1973–2006 in Sweden, 1987–2007 in Finland. Apgar score at 5 minutes: 1978–2008 in Denmark, 1973–2006 in Sweden, 1987–1989, 2003–2007 in Finland; Maternal education available period: 1980–2007 in Denmark, 1990, 1995, 2000, 2005 in Sweden, and 1987–2007 in Finland; Maternal smoking during pregnancy available period: 1991–2007 in Denmark, 1982–2006 in Sweden, 1987–2007 in Finland; Maternal social status available period: 1980–2008 in Denmark, 1980, 1985, 1990 in Sweden, 1990–2007 in Finland.*p