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Table 1 Variables analyzed, and rates of obstetric anal sphincter injury (OASI) in each variable category: singleton, live, term, vaginal 1st, 2nd and 3rd births, Sweden 1999–2011

From: Risk of obstetric anal sphincter injury increases with maternal age irrespective of parity: a population-based register study

Variables 1st births 2nd births 3rd births
Prevalence of variables Rates of OASI Prevalence of variables Rates of OASI Prevalence of variables Rates of OASI
n (436670) % (%) n (379787) % (%) n (140102) % (%)
Maternal age (years)
  < 25 111,828 25.6 4.1 38,677 10.2 1.0 5232 3.7 0.4
 25–29 163,471 37.4 6.9 117,498 30.9 2.0 29,102 20.8 0.6
 30–34 121,833 27.9 8.0 153,263 40.4 2.7 57,036 40.7 0.8
  ≥ 35 39,538 9.1 7.8 70,349 18.5 3.0 48,732 34.8 1.1
Year of birth
 1999–2004 185,654 42.5 7.0 161,407 42.5 2.5 62,560 44.7 1.0
 2005–2011 251,016 57.5 6.3 218,380 57.5 2.2 77,542 55.3 0.8
Education
 Elementary: ≤ 9 years 43,820 10.0 4.2 35,170 9.3 1.6 19,189 13.7 0.7
 High school 175,631 40.2 6.0 163,317 43.0 2.2 63,376 45.2 0.8
 College or university 205,730 47.1 7.6 174,711 46.0 2.6 54,505 38.9 1.0
 Missing 11,489 2.6   6589 1.7   3032 2.2  
Country/region of birth
 Sweden 357,730 81.9 6.6 309,967 81.6 2.4 107,752 76.9 0.9
 Asia 33,177 7.6 7.4 28,907 7.6 2.3 14,270 10.2 0.9
 Africa 7163 1.6 9.0 7171 1.9 3.1 4807 3.4 1.2
 Other 38,600 8.8 5.6 33,742 8.9 1.9 13,273 9.5 0.7
Smoking in early pregnancy
 No 381,092 87.3 6.8 335,534 88.3 2.4 119,673 85.4 0.9
 Yes 33,734 7.7 3.9 25,689 6.8 1.4 13,573 9.7 0.6
 Missing 21,844 5.0   18,564 4.9   6856 4.9  
BMI
 Low: <18.5 12,152 2.8 6.4 7824 2.1 2.1 2015 1.4 0.4
 Normal weight: 18.5–24.9 260,128 59.6 6.5 210,403 55.4 2.2 71,198 50.8 0.9
 Overweight: 25–29.9 83,185 19.0 6.8 84,411 22.2 2.5 34,887 24.9 0.9
 Obese: ≥ 30 31,333 7.2 6.2 35,013 9.2 2.6 15,749 11.2 0.8
 Missing 49,872 11.4   42,136 11.1   16,253 11.6  
Infant birthweight (g)
  < 3000 52,966 12.1 2.7 26,046 6.9 0.9 9447 6.7 0.3
 3000–3499 163,269 37.4 4.5 111,404 29.3 1.3 38,161 27.2 0.4
 3500–3999 157,192 36.0 7.5 150,736 39.7 2.3 54,542 38.9 0.8
 4000–4499 53,342 12.2 11.9 73,289 19.3 3.7 29,503 21.1 1.3
  ≥ 4500 8818 2.0 18.0 17,538 4.6 5.9 8168 5.8 2.6
 Missing 1083 0.2   774 0.2   281 0.2  
Presentation
 Occiput anterior 407,784 93.4 6.4 354,776 93.4 2.3 130,554 93.2 0.8
 Occiput posterior 14,355 3.3 10.8 12,375 3.3 4.0 4645 3.3 1.5
 Breech 1405 0.3 4.3 978 0.3 1.6 327 0.2 0.6
 Other 13,126 3.0 6.9 11,658 3.1 2.6 4576 3.3 1.3
Diabetes
 No 435,364 99.7 6.6 378,146 99.6 2.3 139,359 99.5 0.9
 Yes 1306 0.3 8.7 1641 0.4 5.0 743 0.5 1.5
Gestational diabetes
 No 433,683 99.3 6.6 376,826 99.2 2.3 138,580 98.9 0.9
 Yes 2987 0.7 8.1 2961 0.8 3.3 1522 1.1 1.0
Pregestational hypertension
 No 434,666 99.5 6.6 377,874 99.5 2.3 139,239 99.4 0.9
 Yes 2004 0.5 7.0 1913 0.5 2.1 863 0.6 0.9
Preeclampsia
 No 423,794 97.1 6.6 375,404 98.8 2.3 138,380 98.8 0.9
 Yes 12,876 2.9 6.2 4383 1.2 2.3 1722 1.2 0.9
Epidural analgesia
 No 234,246 53.6 6.1 308,153 81.1 2.0 121,796 86.9 0.8
 Yes 202,424 46.4 7.1 71,634 18.9 3.6 18,306 13.1 1.1
Episiotomy
 No 376,486 86.2 6.2 363,439 95.7 2.2 137,265 98.0 0.8
 Yes 60,184 13.8 8.6 16,348 4.3 5.6 2837 2.0 2.5
Vacum extraction or forceps
 No 362,249 83.0 5.0 364,529 96.0 2.0 136,860 97.7 0.8
 Yes 74,421 17.0 14.4 15,258 4.0 10.6 3242 2.3 4.3
Second births only
History of OASI in 1st pregnancy
 No     320,913 84.5 2.0    
 Yes     16,531 4.4 7.3    
 Missing     42,343 11.1     
History of cesarean section 1st pregnancy
 No     312,869 82.4 1.6    
 Yes     24,575 6.5 10.3    
 Missing     42,343 11.1     
Third births only
History of OASI in 2nd pregnancy
 No        121,980 87.1 0.7
 Yes        1688 1.2 7.6
 Missing        16,434 11.7  
History of cesarean section 2nd pregnancy
 No        121,305 86.6 0.7
 Yes        2363 1.7 2.4
 Missing        16,434 11.7