Table 1. Characteristics of Study Participants by Gestational Age at Birth in Sweden From 1973 to 1994.
| Characteristic | No. (%) | ||||
|---|---|---|---|---|---|
| Early Preterm (<34 wk) (n = 21 748) | Late Preterm (34-36 wk) (n = 80 240) | Early-Term (37-38 wk) (n = 361 132) | Full-Term (39-41 wk) (n = 1 479 457) | Postterm (≥42 wk) (n = 199 132) | |
| Child Characteristics | |||||
| Sex | |||||
| Male | 11 937 (54.9) | 44 065 (54.9) | 189 368 (52.4) | 750 754 (50.7) | 103 679 (52.1) |
| Female | 9811 (45.1) | 36 175 (45.1) | 171 764 (47.6) | 728 703 (49.3) | 95 453 (47.9) |
| Fetal growtha | |||||
| >1 SD below mean | 3322 (15.3) | 10 976 (13.7) | 40 470 (11.2) | 221 399 (15.0) | 57 636 (28.9) |
| −1 to 1 SD from mean | 16 825 (77.4) | 58 828 (73.3) | 260 689 (72.2) | 1 042 526 (70.5) | 123 005 (61.8) |
| >1 SD more than mean | 1601 (7.4) | 10 436 (13.0) | 59 973 (16.6) | 215 532 (14.6) | 18 491 (9.3) |
| Birth order | |||||
| 1 | 10 896 (50.1) | 38 762 (48.3) | 143 759 (39.8) | 606 062 (41.0) | 95 725 (48.1) |
| 2 | 6150 (28.3) | 24 060 (30.0) | 131 049 (36.3) | 557 077 (37.6) | 66 378 (33.3) |
| ≥3 | 4702 (21.6) | 17 418 (21.7) | 86 324 (23.9) | 316 318 (21.4) | 37 029 (18.6) |
| Congenital anomalies | 80 (0.4) | 297 (0.4) | 805 (0.2) | 1721 (0.1) | 303 (0.2) |
| Death during follow-up | 234 (1.1) | 787 (1.0) | 2895 (0.8) | 10 629 (0.7) | 1718 (0.9) |
| Emigration during follow-up | 1003 (4.6) | 4460 (5.6) | 20 842 (5.8) | 92 082 (6.2) | 13 315 (6.7) |
| Maternal Characteristics | |||||
| Age, y | |||||
| <20 | 1396 (6.4) | 4518 (5.6) | 14 674 (4.1) | 58 011 (3.9) | 10 345 (5.2) |
| 20-24 | 5535 (25.5) | 20 867 (26.0) | 86 225 (23.9) | 376 336 (25.4) | 56 203 (28.2) |
| 25-29 | 6861 (31.6) | 27 281 (34.0) | 128 656 (35.6) | 563 933 (38.1) | 75 307 (37.8) |
| 30-34 | 4989 (22.9) | 17 853 (22.3) | 87 648 (24.3) | 345 432 (23.4) | 42 222 (21.2) |
| 35-39 | 2393 (11.0) | 7952 (9.9) | 36 043 (10.0) | 116 842 (7.9) | 13 088 (6.6) |
| ≥40 | 574 (2.6) | 1769 (2.2) | 7886 (2.2) | 18 903 (1.3) | 1967 (1.0) |
| Education, y | |||||
| ≤9 | 4221 (19.4) | 14 654 (18.3) | 59 650 (16.5) | 220 756 (14.9) | 32 723 (16.4) |
| 10-12 | 11 310 (52.0) | 41 120 (51.2) | 181 009 (50.1) | 737 256 (49.8) | 98 740 (49.6) |
| >12 | 6217 (28.6) | 24 466 (30.5) | 120 473 (33.4) | 521 445 (35.3) | 67 669 (34.0) |
| Birth country or region | |||||
| Sweden | 18 862 (86.7) | 70 494 (87.8) | 316 289 (87.6) | 1 323 231 (89.4) | 179 612 (90.2) |
| Other Europe/United States/Canada | 1964 (9.0) | 6626 (8.3) | 28 917 (8.0) | 108 667 (7.4) | 14 311 (7.2) |
| Asia/Oceania | 601 (2.8) | 2122 (2.6) | 11 100 (3.1) | 31 913 (2.2) | 3193 (1.6) |
| Africa | 122 (0.6) | 382 (0.5) | 1726 (0.5) | 6219 (0.4) | 988 (0.5) |
| Latin America | 112 (0.5) | 438 (0.6) | 2478 (0.7) | 7258 (0.5) | 718 (0.4) |
| Other/unknown | 87 (0.4) | 178 (0.2) | 622 (0.2) | 2169 (0.1) | 310 (0.2) |
| Body mass indexb | |||||
| <18.5 | 501 (2.3) | 2564 (3.2) | 11 434 (3.2) | 34 620 (2.3) | 2625 (1.3) |
| 18.5-24.9 | 19 922 (91.6) | 72 157 (89.9) | 322 770 (89.4) | 1 342 188 (90.7) | 183 958 (92.4) |
| 25.0-29.9 | 994 (4.6) | 4267 (5.3) | 21 497 (5.9) | 83 058 (5.6) | 9909 (5.0) |
| ≥30.0 | 331 (1.5) | 1252 (1.6) | 5431 (1.5) | 19 591 (1.3) | 2640 (1.3) |
| Smoking, cigarettes/d | |||||
| 0 | 12 683 (58.3) | 48 168 (60.0) | 229 370 (63.5) | 940 845 (63.6) | 116 006 (58.3) |
| 1-9 | 7163 (32.9) | 25 978 (32.4) | 106 921 (29.6) | 460 344 (31.1) | 75 440 (37.9) |
| ≥10 | 1902 (8.8) | 6094 (7.6) | 24 841 (6.9) | 78 268 (5.3) | 7686 (3.9) |
| Preeclampsia | 3123 (14.4) | 7978 (9.9) | 22 988 (6.4) | 66 241 (4.5) | 9491 (4.8) |
| Other hypertensive disorders | 297 (1.4) | 824 (1.0) | 3032 (0.8) | 8295 (0.6) | 806 (0.4) |
| Diabetes | 198 (0.9) | 933 (1.2) | 2585 (0.7) | 2874 (0.2) | 141 (0.1) |
a
Fetal growth was defined as number of SDs from mean birth weight for gestational age and sex based on Swedish reference intrauterine growth curves.
b
Calculated as weight in kilograms divided by height in meters squared.