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Table 2 Prevalence (%) of OASIS in relation to birth position

From: Birth position and obstetric anal sphincter injury: a population-based study of 113 000 spontaneous births

   Total Sitting Lithotomy Lateral Knee Birth seat Supine Squatting Standing All fours Unknown
Nulliparous women N of OASIS 2 548 931 850 250 126 279 58 15 9 13 17
OASIS (%) 5.7 5.4 7.1 4.3 4.7 5.7 4.8 4.4 3.7 5.5 6.0
 3rd degree (%) 5.2 4.9 6.5 4.0 4.3 5.1 4.4 3.8 3.7 5.5 5.5
 4th degree (%) 0.5 0.5 0.6 0.3 0.4 0.6 0.4 0.6 0.0 0.0 0.5
Parous women N of OASIS 859 323 194 124 70 82 37 10 5 6 8
OASIS (%) 1.3 1.2 2.6 1.0 0.9 1.6 1.0 2.1 0.6 1.4 1.8
 3rd degree (%) 1.1 1.1 2.0 0.9 0.8 1.4 0.9 1.7 0.6 1.2 1.7
 4th degree (%) 0.2 0.1 0.6 0.1 0.1 0.2 0.1 0.4 0.0 0.2 0.1
Women undergoing VBAC N of OASIS 301 100 106 32 16 27 13 3 1 1 2
OASIS (%) 10.6 9.5 12.3 8.3 10.7 12.0 18.2 14.3 5.6 6.3 11.8
 3rd degree (%) 10.2 9.5 10.9 7.4 10.7 11.6 18.2 14.3 5.6 1.2 11.8
 4th degree (%) 0.6 0.0 1.4 1.0 0.0 0.4 0.0 0.0 0.0 0.2 0.0