OASIS OASIS All vaginal births Crude Crude
n (%) n RR (95%CI)
Year of delivery 2005–2008 12 226 (3.6) 335 805 1.0 Reference
2009–2012 12 370 (3.5) 357 174 0.95 0.93–0.98
2013–2016 9987 (3.4) 296 009 0.93 0.90–0.95
Maternal age (years) <20 349 (2.3) 15 423 0.59 0.53–0.66
20–24 3959 (3.0) 133 787 0.77 0.74–0.80
24–29 11 522 (3.8) 300 127 1.0 Reference
30–34 13 037 (3.8) 341 750 0.99 0.97–1.02
34–39 4919 (3.0) 165 330 0.78 0.75–0.80
≥40 797 (2.4) 32 571 0.64 0.59–0.68
Parity 1 25 576 (6.0) 429 727 2.81 2.74–2.88
2 7871 (2.1) 371 933 1.0 Reference
≥3 1136 (0.6) 187 338 0.29 0.27–0.31
Smoking Yes 1077 (1.8) 60340 0.49 0.47-0.52
No 31891 (3.6) 882298 1.0 Reference
Not known 1615 (3.5) 46360 0.96 0.92-1.01
Body Mass Index (kg/m2) <18.5 951 (4.0) 23543 1.12 1.05-1.19
20-24.9 20428 (3.6) 563735 1.0 Reference
25-29.9 7563 (3.4) 222729 0.94 0.91-0.96
≥30 3022 (2.9) 103740 0.80 0.77-0.84
Not known 2600 (3.5) 74627 0.96 0.92-1.00
Maternal height (cm) <155 1159 (4.2) 27892 1.21 1.14-1.28
155-164 11704 (3.6) 325729 1.05 1.02-1.07
165-174 16745 (3.4) 488258 1.0 Reference
≥175cm 3332 (3.3) 99815 0.97 0.94-1.01
Not known 1643 (3.5) 47298 1.01 0.96-1.07
Maternal educational level ≤Primary/lower secondary 1972 (2.3) 86794 0.55 0.52-0.57
Upper secondary 9675 (2.9) 333887 0.70 0.68-0.72
<3 yrs higher education 4280 (3.6) 119622 0.86 0.83-0.89
≥3 yrs higher education 15999 (4.2) 385437 1.00 Reference
Not known 2657 (4.2) 63258 1.01 0.97-1.05
Maternal country of birth Nordic countries 30112 (3.5) 860890 1.00 Reference
Europe/USA/ Canada/Australia/NZ 698 (3.4) 20409 0.98 0.97-1.04
Other 3773 (3.5) 107699 1.00 0.97-1.05
Delivery mode Non-instrumental 23946 (2.6) 905290 1.00 Reference
VE 10314 (12.6) 81932 4.76 4.66-4.87
Forceps 323 (18.2) 1776 6.88 6.23-7.59