Fig. 1.

Flowchart illustrating the selection of the singletons under study and the exclusion due to missing values. ^a Including registration of gestational age <20 weeks (n = 95, range 4-19 weeks) and >45 weeks (n = 22, range 46-55 weeks). ^b Distribution of missing values: 1978 (n = 18,218), 1979 (n = 15,111), 1980 (n = 11,187), 1981 (n = 6,683), 1982-2006 (n = 9,775). ^c Including registration of birth weight <500 g (n = 122, range 3-496 g), >6,000 g (n = 657, range 6,004-7,253 g) and birth weight registration considered too large for gestational week 20-33 (n = 582).