Table 1

Descriptive statistics for preterm infants born ≤32⁺⁶ weeks gestation by neonatal unit characteristic at the hospital of birth

	Designation of unit			Volume of unit*
	Tertiary-level unit	Non-tertiary- level unit	p Value†	High- volume unit	Non-high- volume unit	p Value†
n (%)	9466 (46.1)	11 088 (54.0)		9541 (46.4)	11 013 (53.6)
Gestation (weeks), mean (SD)	29.2 (2.5)	30.0 (2.1)	<0.001	29.3 (2.5)	29.9 (2.2)	<0.001
Birth weight (g), mean (SD)	1313.9 (438.7)	1451.9 (404.5)	<0.001	1326.6 (436.7)	1441.8 (409.4)	<0.001
Received full or partial course of antenatal steroids	6394 (67.6)	7262 (65.5)	0.002	6330 (66.4)	7326 (66.5)	0.790
Deprivation score bottom 10%	2020 (21.4)	1342 (12.1)	<0.001	1730 (18.1)	1632 (14.8)	<0.001
Male	5048 (53.3)	5397 (53.4)	0.756	5093 (53.4)	5892 (53.5)	0.863
Neonatal mortality	423 (4.5)	366 (3.3)	<0.001	394 (4.1)	395 (3.6)	0.043
Any in-hospital mortality	569 (6.0)	425 (3.8)	<0.001	527 (5.5)	467 (4.2)	<0.001
BPD‡	3695 (39.0)	2856 (25.8)	<0.001	3548 (37.2)	3003 (27.3)	<0.001
Treatment for ROP	226 (2.4)	107 (1.0)	<0.001	195 (2.0)	138 (1.3)	<0.001
Surgery for NEC	167 (1.8)	123 (1.1)	<0.001	163 (1.7)	127 (1.2)	0.001
PMA§ at discharge >40⁺⁰ weeks	1292 (13.7)	848 (7.7)	<0.001	1237 (13.0)	903 (8.2)	<0.001

All values are n (%) unless otherwise stated.
*High volume was defined as being in the top quartile of units by number of care days provided to infants born at ≤32⁺⁶ weeks gestation.
†Continuous variables were tested by t test, categorical variables by χ² test.
‡BPD defined as requirement of supplementary oxygen for at least 28 days postbirth and at 36 weeks postmenstrual age.
§PMA at discharge equal to gestational age at birth plus length of stay in weeks.
BPD, bronchopulmonary dysplasia; NEC, necrotising enterocolitis; PMA, postmenstrual age; ROP, retinopathy of prematurity.