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Table 1 Parental socio-demographic characteristics, year of birth and birth characteristics of the studied twins and singletons, by gender

From: Reproductive patterns among twins - a Swedish register study of men and women born 1973-1983

  Females   Males  
  Singletons Twins   Singletons Twins  
  n % n % p-value n % n % p-value
Mother’s educational level <0.001      <0.001
 Missing* 32 269 6.6 533 6.5   33 850 6.6 567 6.8  
 9-10 years 139 153 28.6 2371 28.7   146 638 28.5 2331 28.0  
 11-13 years 205 252 42.2 3388 41.1   216 573 42.2 3338 40.2  
 ≥14 years 109 652 22.6 1956 23.7   116 740 22.7 2077 25.0  
Father’s educational level <0.001      <0.001
 Missing* 36 483 7.5 534 6.5   38 355 7.5 605 7.3  
 9-10 years 153 488 31.6 2625 31.8   161 647 31.5 2567 30.9  
 11-13 years 193 682 39.8 3247 39.4   204 833 39.9 3238 38.9  
 ≥14 years 102 673 21.1 1842 22.3   108 966 21.2 1903 22.9  
Mother’s civil status <0.001      <0.001
 Divorced/widowed 16 938 3.5 349 4.2   17 752 3.5 378 4.5  
 Unmarried 132 503 27.2 1960 23.8   139 369 27.1 1953 23.5  
 Married 336 885 69.3 5939 72.0   356 680 69.4 5982 72.0  
Parental country of origin 0.971      0.930
 One or both non-Nordic 37 553 7.7 636 7.7   39 300 7.7 638 7.7  
 Both Nordic 448 773 92.3 7612 92.3   474 501 92.3 7675 92.3  
Mother’s age at birth <0.001      <0.001
 13-19 years 27 291 5.6 284 3.4   28 543 5.6 285 3.4  
 20-26 years 215 608 44.3 3137 38.0   227 765 44.3 3164 38.1  
 27-33 years 194 398 40.0 3726 45.2   205 474 40.0 3748 45.1  
 ≥34 years 49 029 10.1 1101 13.3   52 019 10.1 1116 13.4  
Mother’s parity <0.001      <0.001
 Previous children 278 112 57.2 5434 65.9   294 131 57.2 5659 68.1  
 No previous children 208 214 42.8 2814 34.1   219 670 42.8 2654 31.9  
Year of birth <0.001      <0.001
 Mean/SD 1977.82/3.18 1978.00/3.16   1977.82/3.18 1978.05/3.19  
Size for gestational age <0.001      <0.001
 Appropriate for gestational age 455 865 93.7 6228 75.5   481 038 93.6 6652 80.0  
 Small for gestational age 18 573 3.8 1985 24.1   18 213 3.5 1614 19.4  
 Large for gestational age 11 888 2.4 35 0.4   14 550 2.8 47 0.6  
Birthweight <0.001      <0.001
 Normal birthweight (> = 2500 g) 470 607 96.8 4860 58.9   499971 97.3 5411 65.1  
 Low birthweight (1500–2499 g) 14 661 3.0 3186 38.6   12840 2.5 2725 32.8  
 Very low birthweight (<1500 g) 1058 0.2 202 2.4   990 0.2 177 2.1  
Gestational age <0.001      <0.001
 Term (37–42 weeks) 455 742 93.7 5406 65.5   478 279 93.1 5400 65.0  
 Preterm (32–36 weeks) 17 399 3.6 2548 30.9   21 909 4.3 2578 31.0  
 Very preterm (<32 weeks) 1539 0.3 251 3.0   1779 0.3 294 3.5  
 Post term (>42 weeks) 11 646 2.4 43 0.5   11 834 2.3 41 0.5  
  1. *Parents with missing information on education were mainly immigrants and therefore included as a separate category in the analysis.