Table 2.

Relationship between covariates and bipolar disorder in case and control subjects.

Covariates	Controls		Cases			p-value
	Mean	SD	Mean	SD	t
Maternal age (years)	28.80	4.96	28.28	5.68	1.33	0.18
Gestational week of blood draw	10.75	3.61	10.85	3.83	−0.37	0.71
	N	%	N	%	χ2
Previous birthsa					2.82	0.09
0	146	39.6	169	45.7
≥ 1	223	60.4	201	54.3
Parental BPD historyb					31.63	<0.0001
No	369	98.7	327	88.6
Yes	5	1.3	42	11.4
Parental psychiatric history					62.18	<0.0001
No	304	80.4	202	53.4
Yes	74	19.6	176	46.6
Maternal smoking during pregnancyc					5.34	0.02
No	304	84.9	288	78.3
Yes	54	15.1	80	21.7
Maternal education level					0.40	0.81
University degree	26	6.9	24	6.3
Upper secondary school	268	70.9	263	69.6
Less than high school	84	22.2	91	24.1
Province of birth					12.44	0.006
Southern Finland	134	35.4	168	44.4
Eastern Finland	45	11.9	52	13.8
Western Finland	136	36.0	93	24.6
Northern Finland	63	16.7	65	17.2

^aData missing for 8 cases and 9 controls

^bData missing for 9 cases and 4 controls

^cData missing for 10 cases and 20 controls

SD: standard deviation; t: t-test value; χ²: Pearson’s chi squared test value