Table 2.

Cox regression showing the hazard ratio (HR) of vaginal birth in the defined body mass index (BMI) groups, with and without adjustment for covariates

	Total duration a
Covariates	Crude HR	95% CI	Adjusted HR	95% CI
BMI
< 25	1		1
25–29.9	0.83	[0.73–0.96]	1.01	[0.88–1.16]
≥ 30	0.94	[0.78–1.12]	1.07	[0.90–1.28]
Height
< 160 cm	1		1
160–169 cm	1.31	[1.08–1.59]	1.32	[1.09–1.61]
170–179 cm	1.44	[1.18–1.76]	1.61	[1.32–1.98]
≥ 180 cm	1.66	[1.21–2.26]	1.73	[1.26–2.38]
Age
< 25	1		1
25–29	0.87	[0.77–0.99]	1.04	[0.91–1.19]
30–34	0.82	[0.72–0.94]	0.97	[0.84–1.11]
≥ 35	0.62	[0.51–0.77]	0.68	[0.54–0.84]
Birthweight
< 3.0 kg	1		1
3.0–3.5 kg	0.77	[0.66–0.90]	0.66	[0.56–0.78]
3.5–4.0 kg	0.53	[0.46–0.62]	0.49	[0.42–0.58]
> 4.0 kg	0.38	[0.31–0.46]	0.38	[0.30–0.47]
Epidural
No	1		1
Yes	0.29	[0.26–0.32]	0.33	[0.29–0.37]
Augmentation with oxytocin:
No	1		1
Yes	0.31	[0.28–0.35]	0.40	[0.36–0.45]
Induced labour:
No	1		1
Yes	0.91	[0.82–1.02]	1.68	[1.48–1.90]
Phase 1
	Crude HR	95% CI	Adjusted HR	95% CI
BMI
< 25	1		1
25–29.9	0.99	[0.86–1.14]	1.11	[0.96–1.28]
≥ 30	1.01	[0.85–1.21]	1.04	[0.86–1.24]
Phase 2
	Crude HR	95% CI	Adjusted HR	95% CI
BMI
< 25	1		1
25–29.9	0.90	[0.96–1.28]	0.99	[0.86–1.14]
≥ 30	1.06	[0.96–1.28]	1.29	[1.07–1.55]

Phase 1 was defined as the period of time from active labour until the cervix was fully dilated

Phase 2 was defined as the period of time from when the cervix was fully dilated to the actual birth. Caesarean deliveries censored

A hazard ratio (HR) >1 indicates an increased number of vaginal deliveries over time compared to the reference group; a shorter duration of labour. A HR < 1 indicates a decreased number of vaginal deliveries over time compared to the reference group; a longer duration of labour

^a77 women (4.1%) did not have duration measures recorded and were therefore not accounted for in analysis