Table 3.

Relationship between covariates and ADHD in case and control subjects

Covariates	Cases		Controls		t	p value
	Mean	SD	Mean	SD
Maternal age (years)	27.89	5.87	29.55	5.31	6.73	< 0.001
Gestational week of blood draw	10.72	3.64	10.63	3.08	− 0.61	0.54

Covariates	Cases		Controls		χ2	p value
	N	%	N	%
Previous births					14.69	< 0.001
0	486	47.37	400	38.99
≥ 1	540	52.63	626	61.01
History of maternal ADHD diagnosisa					4.51	0.03
No	1019	99.32	1025	99.90
Yes	7	0.68	1	0.10
History of maternal substance abuseb					26.09	< 0.001
No	970	94.54	1012	98.64
Yes	56	5.46	14	1.36
History of maternal psychopathologyc					60.25	< 0.001
No	766	74.66	903	88.01
Yes	260	25.34	123	11.99
History of paternal ADHD diagnosisa*					6.58	0.01
No	991	99.10	1018	99.90
Yes	9	0.90	1	0.10
History of paternal psychopathologyd*					41.82	< 0.001
No	755	75.50	884	86.75
Yes	245	24.50	135	13.25
Maternal smoking1					83.31	< 0.001
No	701	70.17	866	86.95
Yes	298	29.83	130	13.05
Maternal SES					35.19	< 0.001
Upper white collar	71	6.92	138	13.45
Lower white collar	408	39.77	447	43.57
Blue collar	214	20.86	183	17.84
Others	205	19.98	159	15.50
Missing	128	12.48	99	9.65
Gestational age (weeks)					25.07	< 0.001
< 37	98	9.59	41	4.01
≥ 37	924	90.41	981	95.99
Weight for gestational age					11.14	0.003
< − 2 SD	51	5.00	23	2.25
− 2 SD to + 2 SD	936	91.67	961	94.03
> + 2 SD	34	3.33	38	3.72
Maternal immigration status					6.63	0.01
No	1004	97.86	1018	99.22
Yes	22	2.14	8	0.78

SD standard deviation, t t test value, χ² Pearson’s Chi-squared test value

^aICD-10: F90.X or ICD-9: 314.X

^bICD-8 (291, 303, 304), ICD-9 (291, 292, 303,304,305) or ICD-10 (F10-19)

^cICD-8 (291–308), ICD-9 (291–316) or ICD-10 (F10-99, excluding mental retardation (F70-79) excluding maternal substance abuse diagnosis

^dICD-8 (291–308), ICD-9 (291–316) or ICD-10 (F10-99, excluding mental retardation (F70-79)

Bold values indicate significance p<0.05