Table 1 Clinical characteristics of the eligible population

	Study population (n = 8431)			Not evaluated (n = 6912)
Variables	^aTotal	Median/n	Range/%	^aTotal	Median/n	Range/%	P
^bGestational age [weeks, days] median, range	8424	28w6d	22w0d–39w1d	6905	29w0d	21w2d–40w1d	< *0.01*
^bBirth weight [grams] median, range	8431	1066	267–1500	6911	1108	308–1500	< *0.01*
Maternal age of 35 or more years old	7937	2316	29.2	6522	1851	28.4	0.29
Multiple birth	8431	2128	25.2	6912	1748	25.3	0.94
Chorioamnionitis	8345	2250	27.0	6796	1656	24.4	< *0.01*
Antenatal steroid	8409	3857	45.9	6885	2704	39.3	< *0.01*
Cesarean section	8427	6733	79.9	6901	5236	75.9	< *0.01*
Male	8423	4337	51.5	6909	3542	51.3	0.78
Apgar score of less than 7-point at 5-minutes after birth	7914	1459	18.4	6353	1036	16.3	< *0.01*
Small-for-gestational age	8331	3333	40.0	6867	2621	38.2	*0.02*
Respiratory distress syndrome	8415	4736	56.3	6805	3387	49.8	< *0.01*
Moderate-to severe bronchopulmonary dysplasia	3133	1587	50.7	2258	1072	47.5	*0.02*
Sepsis	8411	590	7.0	6796	358	5.3	< *0.01*
Symptomatic patent ductus arteriosus	7093	3207	45.2	5252	2401	45.7	0.58
Severe intraventricular hemorrhage	8425	231	2.7	6895	240	3.5	*0.01*
Cystic periventricular leukomalacia	8424	255	3.0	6885	237	3.4	0.15
Necrotizing enterocolitis	8418	59	0.7	6798	70	1.0	*0.03*
Treating retinopathy of prematurity	8292	1247	15.0	6496	780	12.0	< *0.01*
Cerebral palsy	8317	683	8.2	2090	176	8.4	0.76
Developmental quotient of less than 70	7478	1114	14.9	1897	311	16.4	0.10

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