Table 1. Characteristics of Preterm Birthsa.
| Characteristic | Extremely preterm | Very preterm | Moderately preterm | |||
|---|---|---|---|---|---|---|
| No. (%) | Fetal weeks of follow-up, millions | No. (%) | Fetal weeks of follow-up, millions | No. (%) | Fetal weeks of follow-up, millions | |
| Total preterm births, No. | 2009 | 6.77 | 4746 | 4.50 | 31 384 | 5.56 |
| Maternal employment status | ||||||
| Employed | 1124 (55.9) | 3.87 | 2691 (56.7) | 2.57 | 17 950 (57.2) | 3.17 |
| Employed, management position | 203 (10.1) | 0.96 | 527 (11.1) | 0.64 | 3452 (11.0) | 0.79 |
| Self-employed | 34 (1.7) | 0.14 | 102 (2.1) | 0.10 | 660 (2.1) | 0.12 |
| Unemployed and receiving public assistance | 648 (32.3) | 1.80 | 1426 (30.0) | 1.20 | 9322 (29.7) | 1.48 |
| Family structure | ||||||
| Married | 742 (36.9) | 2.74 | 1664 (35.1) | 1.82 | 10 912 (34.8) | 2.25 |
| Single | 473 (23.5) | 1.20 | 1070 (22.5) | 0.80 | 6686 (21.3) | 0.98 |
| Living with partner | 794 (39.5) | 2.83 | 2012 (42.4) | 1.88 | 13 786 (43.9) | 2.32 |
| Maternal educational level | ||||||
| Primary school | 583 (29.0) | 1.36 | 1270 (26.8) | 0.90 | 7942 (25.3) | 1.11 |
| Secondary school | 212 (10.6) | 0.85 | 500 (10.5) | 0.56 | 3622 (11.5) | 0.70 |
| Postsecondary school | 620 (30.9) | 2.59 | 1551 (32.7) | 1.72 | 10 057 (32.0) | 2.13 |
| Vocational school | 594 (29.6) | 1.98 | 1425 (30.0) | 1.32 | 9763 (31.1) | 1.62 |
| Household disposable income quartile | ||||||
| 1 | 673 (33.5) | 1.73 | 1447 (30.5) | 1.15 | 9161 (29.2) | 1.41 |
| 2 | 633 (31.5) | 2.17 | 1567 (33.0) | 1.44 | 10 587 (33.7) | 1.78 |
| 3 | 471 (23.4) | 1.80 | 1143 (24.1) | 1.20 | 7570 (24.1) | 1.48 |
| 4 | 232 (11.5) | 1.08 | 589 (12.4) | 0.72 | 4066 (13.0) | 0.89 |
| Maternal place of birth | ||||||
| Denmark | 1698 (84.5) | 5.95 | 4165 (87.8) | 3.95 | 27 731 (88.4) | 4.88 |
| Europe | 118 (5.9) | 0.36 | 234 (4.9) | 0.24 | 1468 (4.7) | 0.30 |
| Other | 193 (9.6) | 0.47 | 347 (7.3) | 0.31 | 2185 (7.0) | 0.38 |
| Year of pregnancy onset | ||||||
| 1997-2000 | 341 (17.0) | 1.40 | 1047 (21.4) | 0.93 | 6392 (20.4) | 1.15 |
| 2001-2004 | 415 (20.7) | 1.41 | 1064 (22.4) | 0.93 | 6944 (22.1) | 1.15 |
| 2005-2008 | 431 (21.5) | 1.41 | 974 (20.5) | 0.94 | 6598 (21.0) | 1.16 |
| 2009-2012 | 401 (20.0) | 1.30 | 892 (18.8) | 0.87 | 5830 (18.6) | 1.07 |
| 2013-2016 | 421 (21.0) | 1.16 | 769 (16.2) | 0.79 | 5620 (17.9) | 1.00 |
| Maternal age at pregnancy onset, y | ||||||
| <18 | 520 (25.9) | 1.58 | 1271 (26.8) | 1.05 | 8377 (26.7) | 1.29 |
| 18-24 | 400 (19.9) | 1.55 | 968 (20.4) | 1.03 | 7014 (22.3) | 1.27 |
| 25-34 | 252 (12.5) | 1.10 | 658 (13.9) | 0.73 | 4617 (14.7) | 0.90 |
| 35-44 | 363 (18.1) | 1.32 | 880 (18.5) | 0.88 | 5396 (17.2) | 1.08 |
| ≥45 | 474 (23.6) | 1.23 | 969 (20.4) | 0.82 | 5980 (19.1) | 1.01 |
| BMI | ||||||
| <18.5 | 63 (3.1) | 0.18 | 158 (3.3) | 0.12 | 1112 (3.5) | 0.15 |
| 18.5-25.0 | 670 (33.3) | 2.60 | 1480 (31.2) | 1.73 | 11 187 (35.6) | 2.14 |
| >25.0 | 492 (24.5) | 1.40 | 1010 (21.3) | 0.93 | 6434 (20.5) | 1.15 |
| Unknown | 784 (39.0) | 2.59 | 2098 (44.2) | 1.72 | 12 651 (40.3) | 2.12 |
| Smoking status | ||||||
| Nonsmoker | 1291 (64.3) | 5.23 | 3103 (65.4) | 3.48 | 22 220 (70.8) | 4.30 |
| Stopped smoking during pregnancy | 53 (2.6) | 0.16 | 101 (2.1) | 0.11 | 789 (2.5) | 0.13 |
| Smoker | 359 (17.9) | 0.84 | 882 (18.6) | 0.56 | 5359 (17.1) | 0.69 |
| Unknown | 306 (15.2) | 0.54 | 660 (13.9) | 0.36 | 3016 (9.6) | 0.44 |
Abbreviations: BMI, body mass index (calculated as weight in kilograms divided by height in meters squared); NA, not applicable.
a
Among 1 136 143 pregnancies in Denmark with onset between January 1, 1997, and December 31, 2016.