Table 1 Unadjusted/Adjusted Odds Ratios (OR) of OASR among vaginally delivered women (n = 514,741)

Mode of delivery

Normal vaginal birth

1

Forceps

9.48 (6.18-14.55)

22.55 (9.05-56.22)

≤ 0.001

Vacuum assistance

5.40 (4.98-5.86)

7.37 (6.26-8.67)

≤ 0.001

Episiotomy

2.54 (2.35-2.73)

1.95 (1.77-2.13)

≤ 0.001

Episiotomy and vaginal delivery

1

Episiotomy and forceps

3.02 (2.41-3.80)

0.21 (0.07-0.59)

0.003

Episiotomy and vacuum assistance

2.34 (2.24-2.44)

0.40 (0.33-0.49)

≤ 0.001

Maternal age (years)

≤ 19

1

20-29

1.29 (1.02-1.64)

1.39 (1.09-1.77)

0.008

30-39

1.18 (0.92-1.30)

1.37 (1.07-1.74)

0.01

≥ 40

0.83 (0.59-1.17)

1.01 (0.71-1.44)

0.95

Birth weight (g)

≤ 2999

1

3000-3499

1.69 (1.42-2.02)

1.66 (1.40-1.98)

≤ 0.001

3500-3999

2.40 (2.02-2.84)

2.36 (2.00-2.80)

≤ 0.001

≥ 4000

2.87 (2.72-3.86)

3.22 (2.70-3.83)

≤ 0.001

Amniotomy

0.95 (0.89-1.03)

0.91 (0.84-0.98)

0.01

Augmentation with oxytocin

1.89 (1.75-2.04)

1.12 (1.07-1.27)

≤ 0.001

Epidural analgesia

1.96 (1.82-2.11)

1.37 (1.26-1.49)

≤ 0.001

*Spinal analgesia

0.68 (0.54-0.85)

0.77 (0.61-0.96)

0.02

Nitrous oxide gas

1.03 (0.96-1.11)

0.87 (0.80-0.93)

≤ 0.001

*Occiput posterior presentation

7.95 (5.89-10.74)

2.07 (1.52-2.81)

≤ 0.001

Time of day

8-23.59

1

00.00-7.59

0.85 (0.78-0.92)

0.89 (0.82-0.97)

0.01

Months

All months excluding July

1

July

0.86 (0.75-0.98)

0.85 (0.74-0.97)

0.02