Baseline characteristics of the study groups
| Characteristic | No. (%)* of singleton births, no HDP n = 1 431 651 | No. (%)* of singleton births, with HDP n = 98 631 | No. (%)* of twin births, no HDP n = 21 046 | No. (%)* of twin births, with HDP n = 4283 |
|---|---|---|---|---|
| Age at delivery, mean ± SD, yr | 28.8 ± 5.4 | 29.2 ± 5.5 | 30.7 ± 5.2 | 31.1 ± 5.4 |
| 18–24 | 319 126 (22.3) | 20 519 (20.8) | 2616 (12.4) | 485 (11.3) |
| 25–40 | 1 091 669 (76.3) | 75 871 (76.9) | 17 868 (84.9) | 3615 (84.4) |
| > 40 | 20 856 (1.5) | 2241 (2.3) | 562 (2.7) | 183 (4.3) |
| World region of origin | ||||
| Canada | 1 050 177 (73.4) | 80 537 (81.7) | 16 172 (76.8) | 3518 (82.1) |
| Africa and Middle East | 56 757 (4.0) | 2557 (2.6) | 926 (4.4) | 124 (2.9) |
| Americas | 50 256 (3.5) | 3149 (3.2) | 623 (3.0) | 118 (2.8) |
| Asia and Pacific | 207 322 (14.5) | 8961 (9.1) | 2280 (10.8) | 362 (8.5) |
| Europe | 60 521 (4.2) | 2969 (3.0) | 925 (4.4) | 139 (3.2) |
| United States | 6541 (0.5) | 454 (0.5) | 118 (0.6) | 22 (0.5) |
| Not stated | 77 (0.0) | ≤ 5§ | ≤ 5§ | ≤ 5§ |
| Neighbourhood-level household income | ||||
| Quintile 1 (lowest) | 337 527 (23.6) | 22 028 (22.3) | 3938 (18.7) | 714 (16.7) |
| Quintile 2 | 299 580 (20.9) | 20 795 (21.1) | 4126 (19.6) | 800 (18.7) |
| Quintile 3 | 286 262 (20.0) | 20 576 (20.9) | 4226 (20.1) | 904 (21.1) |
| Quintile 4 | 280 197 (19.6) | 19 793 (20.1) | 4659 (22.1) | 963 (22.5) |
| Quintile 5 (highest) | 221 176 (15.4) | 14 939 (15.1) | 4027 (19.1) | 882 (20.6) |
| Unknown | 6909 (0.5) | 500 (0.5) | 70 (0.3) | 20 (0.5) |
| Rural residence at delivery | 78 470 (5.5) | 6481 (6.6) | 1107 (5.3) | 238 (5.6) |
| Prepregnancy morbidity | ||||
| Diabetes | 13 902 (1.0) | 2759 (2.8) | 280 (1.3) | 101 (2.4) |
| Chronic hypertension | 39 389 (2.8) | 9388 (9.5) | 702 (3.3) | 312 (7.3) |
| Tobacco use or drug dependence | 50 694 (3.5) | 3767 (3.8) | 749 (3.6) | 147 (3.4) |
| Kidney disease | 4110 (0.3) | 622 (0.6) | 71 (0.3) | 30 (0.7) |
| Assisted reproductive technology | 11 980 (0.8) | 1387 (1.4) | 2596 (12.3) | 613 (14.3) |
| Gestational age at birth, wk | ||||
| < 37 | 64 071 (4.5) | 13 666 (13.9) | 10 081 (47.9) | 2514 (58.7) |
| < 34 | 17 867 (1.2) | 5060 (5.1) | 4008 (19.0) | 594 (13.9) |
| Neonatal characteristics† | ||||
| Female infant | 698 713 (48.8) | 47 191 (47.8) | 20 649 (49.1) | 4320 (50.4) |
| Birth weight, mean ± SD, g | 3367 ± 545 | 3159 ± 742 | 2358 ± 656 | 2365 ± 543 |
| ≤ Tenth percentile‡ | 104 987 (7.3) | 19 663 (19.9) | 27 377 (65.0) | 6007 (70.1) |
| ≤ Third percentile‡ | 24 470 (1.7) | 8386 (8.5) | 11 975 (28.4) | 2308 (26.9) |
| Duration of follow-up of pregnant person, median (IQR), yr | 13 (7–20) | 13 (7–20) | 12 (7–18) | 12 (7–19) |
Note: HDP = hypertensive disorders of pregnancy, IQR = interquartile range, SD = standard deviation.
↵* Unless otherwise specified.
↵† The unit of analysis for neonatal characteristics is infant rather than pregnancy.
↵‡ Based on the Canadian growth reference of Kramer and colleagues.45
↵§ Data suppressed because of small cell size.