Table 1.
Characteristics of the study population
| Female twins | Male twins | |||
|---|---|---|---|---|
| Characteristics | OSa (n=11,078) | SSa (n=19,186) | OSa (n=11,080) | SSa (n=20,207) |
| N (%) | N (%) | N (%) | N (%) | |
| Zygosity | ||||
| Monozygotic | . | 3,425 (17.9) | . | 3,436 (17.0) |
| Dizygotic | 11,078 (100) | 5,719 (29.8) | 11,080 (100) | 6,030 (29.8) |
| Unknown | . | 10,042 (52.3) | . | 10,741 (53.2) |
| Maternal age at birth (years) | ||||
| <25 | 976 (8.8) | 2,662 (13.9) | 985 (8.9) | 2,727 (13.5) |
| 25-29 | 3,364 (30.4) | 6,514 (34.0) | 3,353 (30.3) | 6,745 (33.4) |
| 30-35 | 4,421 (39.9) | 6,828 (35.6) | 4,421 (39.9) | 7,357 (36.4) |
| 35-39 | 2,029 (18.3) | 2,796 (14.6) | 2,033 (18.3) | 3,021 (15.0) |
| 40+ | 288 (2.6) | 386 (2.0) | 288 (2.6) | 357 (1.8) |
| Paternal age at birth (years) | ||||
| <30 | 2,727 (24.6) | 5,891 (30.7) | 2,728 (24.6) | 6,398 (31.7) |
| 30-39 | 6,883 (62.1) | 11,088 (57.8) | 6,880 (62.1) | 11,468 (56.8) |
| 40+ | 1,374 (12.4) | 2,032 (10.6) | 1,375 (12.4) | 2,091 (10.3) |
| Missing | 94 (0.8) | 175 (0.9) | 97 (0.9) | 250 (1.2) |
| Maternal completed highest education | ||||
| Primary and lower secondary | 1,902 (17.2) | 3,600 (18.8) | 1,914 (17.3) | 3,763 (18.6) |
| Upper and post-secondary | 4,477 (40.4) | 7,801 (40.7) | 4,468 (40.3) | 8,210 (40.6) |
| Tertiary | 4,560 (41.2) | 7,476 (39.0) | 4,558 (41.1) | 7,979 (39.5) |
| Missing | 139 (1.3) | 309 (1.6) | 140 (1.3) | 255 (1.3) |
| Paternal completed highest education | ||||
| Primary and lower secondary | 2,038 (18.4) | 3,612 (18.8) | 2,029 (18.3) | 3,909 (19.3) |
| Upper and post-secondary | 5,218 (47.1) | 9,140 (47.6) | 5,231 (47.2) | 9,485 (46.9) |
| Tertiary | 3,561 (32.1) | 5,867 (30.6) | 3,557 (32.1) | 6,190 (30.6) |
| Missing | 261 (2.4) | 567 (3.0) | 263 (2.4) | 623 (3.1) |
| Calendar year of birth | ||||
| 1977-1984 | 1,234 (11.1) | 2,806 (14.6) | 1,231 (11.1) | 3,249 (16.1) |
| 1985-1994 | 2,451 (22.1) | 4,994 (26.0) | 2,463 (22.2) | 4,953 (24.5) |
| 1995-1999 | 2,212 (20.0) | 3,414 (17.8) | 2,213 (20.0) | 3,768 (18.6) |
| 2000-2004 | 2,553 (23.0) | 3,913 (20.4) | 2,550 (23.0) | 4,114 (20.4) |
| 2005-2009 | 2,628 (23.7) | 4,059 (21.2) | 2,623 (23.7) | 4,123 (20.4) |
aOS opposite-sex twins, i.e., twins with an opposite sex co-twin, SS same-sex twins, i.e., twins with a same sex co-twin