Table 1.

Characteristics and birth data of OASI respondents versus non-respondents and overall hospital maternity population during the study period. Bold P values indicate statistical significance P<0.05

		Respondents n = 210	Non-responders n = 393	P value respondents versus non-respondents	Hospital average 2009–20141 24,576	P value total population versus OASI respondents
Mean age at time of birth (years)		31.6	30.5	0.003	32.4	0.003
Gestation at birth (weeks)		40.0	40.0	0.82	38.8	< 0.001
Parity2	0 1 2+	81% 17% 2%	83% 15% 2%	0.49 0.44 1.0	55% 31% 14%	< 0.001 < 0.001 < 0.001
Australian-born		40%	31%	0.001	48%	0.34
Mean birth weight (g)		3577	3530	0.26	3273	< 0.001
Head circumference (cm)		34.6	34.6	0.76	34.1	< 0.001
Mean Apgar 5 min		8.9	8.9	0.60	8.8	0.11
Mode of birth3	NVB	49%	55%	0.13	75%	< 0.001
	Vacuum	14%	15%	0.82	11%	< 0.001
	Forceps	36%	30%	0.14	14%	< 0.001
	Breech	1%	0.3%	0.12	1%	0.41
Episiotomy rate4		47%	38%	0.04	24%	< 0.001
Shoulder dystocia4		12%	13%	0.71	5%	< 0.001
Instrumental level	Outlet Low cavity Mid-cavity High	(n = 105) 9% 29% 63% 0%	(n = 176) 10% 31% 60% 0%	0.08 0.71 0.60	(n = 4221) 10% 36% 53% 0.2%	0.44 0.12 0.048 1.0
Grade of OASI5	3a	51%	56%	0.34	55%	0.33
	3b	33%	30%	0.52	31%	0.52
	3c	7%	8%	0.66	7%	0.96
	4	7%	5%	0.24	7%	0.86
	Not coded	1%	1%	1.0	< 1%	1.0

¹2011 data not available: table shows aggregate of 2009, 2010, 2012–2014 (n = 20,151 births)

²Parity prior to OASI birth

³Total RHW births: Percentage in this table is percentage of vaginal births. For all births 2009–2014 percentages were NVB 52.3%, vacuum 7.6%, forceps 9.5%, vaginal breech 0.7% and cesarean 29.9%

⁴In vaginal births

⁵As proportion of total OASI. For overall hospital population, OASI rate 2009–2014 was: 3a tear 1.5% (2.1% of vaginal births), 3b 0.8% (1.2%), 3c 0.2% (0.3%) and 4th degree 0.2% (0.3%) for a total OASI rate of 2.7% (3.8% of vaginal births)