Descriptive statistics of the study population
| Variables | Exposed cohort (n=1 426 013) n (%) | Unexposed cohort (n=6 123 531) n (%) | p Value |
|---|---|---|---|
| Sex | |||
| Male | 773 690 (51) | 3 143 231 (51) | |
| Female | 732 244 (49) | 2 980 280 (49) | |
| Missing | 4 (<1) | 20 (<1) | 0.564 |
| Birth weight* (g) | |||
| <2500 | 68 125 (5) | 232 387 (4) | |
| 2500–3999 | 1 119 223 (77) | 4 024 314 (75) | |
| ≥4000 | 250 538 (17) | 914 659 (17) | |
| Missing | 22 344 (2) | 215 793 (4) | <0.01 |
| Maternal age | |||
| ≤26 | 519 495 (35) | 2 398 883 (39) | |
| 27–30 | 443 592 (30) | 1 777 990 (29) | |
| ≥31 | 542 782 (36) | 1 946 022 (32) | |
| Missing | 69 (<1) | 636 (<1) | <0.01 |
| Gestational age (weeks)* | |||
| <37 | 82 407 (6) | 82 407 (6) | |
| ≥37 | 1 351 860 (93) | 1 351 860 (93) | |
| Missing | 25 961 (2) | 25 961 (2) | <0.01 |
| Parity† | |||
| 1 | 582 558 (41) | 2 648 077 (46) | |
| 2 | 519 142 (37) | 2 030 259 (35) | |
| ≥3 | 305 963 (22) | 1 059 912 (18) | |
| Missing | 806 (<1) | 10 633 (<1) | <0.01 |
| Maternal education‡ (years) | |||
| Low, ≤9 | 701 579 (49) | 2 306 817 (44) | |
| Middle, 10–14 | 398 797 (28) | 1 461 584 (28) | |
| High, ≥15 | 296 247 (21) | 1 046 192 (20) | |
| Missing | 44 454 (3) | 442 534 (8) | <0.01 |
| Maternal smoking during pregnancy§ | |||
| Yes | 222 980 (26) | 703 266 (19) | |
| No | 572 800 (66) | 2 674 088 (72) | |
| Missing | 69 925 (8) | 315 934 (9) | <0.01 |
| Singleton† | |||
| Yes | 1 437 016 (97) | 5 451 148 (95) | |
| No | 39 129 (3) | 147 173 (3) | |
| Missing | 10 030 (1) | 150 560 (3) | <0.01 |
| Apgar score at 5 min* | |||
| Low, 0–6 | 13 960 (1) | 52 667 (1) | |
| Normal, 7–10 | 1 352 106 (93) | 4 946 721 (92) | |
| Missing | 94 162 (6) | 387 762 (7) | <0.01 |
| Country | |||
| Denmark | 663 820 (44) | 2 728 862 (45) | |
| Sweden | 842 118 (56) | 3 394 669 (55) | <0.01 |
| Maternal origin | |||
| Nordic | 1 482 681 (99) | 5 758 361 (94) | |
| Non-Nordic | 19 532 (1) | 333 804 (6) | |
| Missing | 3725 (<1) | 31 366 (1) | <0.01 |
*Available from 1978 in Denmark and from 1973 in Sweden.
†Available from 1973 in Denmark and from 1973 in Sweden.
‡Available from 1980 in Denmark and from 1973 in Sweden.
§Available from 1991 in Denmark and from 1983 in Sweden.