Table 3.

Proportions (%) of observed confounders by (A) birth order and number of siblings for the full population and (B) age difference with second-born sibling for first-born children. Birth cohorts 1995–2004

	N	Rural residence	Parental tertiary education	Parental above median income	Caesarian section	Parental antihistamines	Parental eczema med.	Parental asthma med.	Parental Epinephrine
A) Birth order (#) by number of children
#1/1	88 481	24.7	40.3	38.2	22.4	65.2	68.1	46.6	4.6
#1/2	112 893	25.1	57.8	54.3	20.5	66.7	68.2	46.6	5.1
#2/2	111 258	27.9	58.1	45.3	15.0	65.7	68.9	46.6	5.0
#1/3	46 389	28.3	56.9	49.1	16.9	64.8	66.4	44.1	4.9
#2/3	48 000	31.8	56.8	40.3	13.9	63.7	66.8	44.6	4.7
#3/3	51 630	36.6	57.4	38.2	14.2	62.7	67.4	44.7	4.6
#1/4+	19 113	33.4	40.1	31.6	12.7	60.4	63.2	39.6	4.3
#2/4+	19 237	37.4	46.4	24.9	8.9	60.0	64.1	40.3	4.4
#3/4+	20 888	41.8	49.1	24.6	11.6	59.6	64.8	40.7	4.5
#4+/4+	41 188	51.1	45.0	15.3	10.7	56.7	66.3	39.7	4.1
Total	559 077	30.8	52.9	41.1	16.5	64.0	67.4	44.9	4.8
B) First-born’s age difference with second-born
Twins	9 237	26.1	57.8	44.3	51.2	66.4	70.0	47.8	5.3
1 year	21 650	30.3	44.6	42.5	15.2	63.8	66.5	44.6	4.8
2 years	66 300	27.3	58.3	53.5	16.2	65.3	67.0	44.8	4.9
3 years	39 629	25.9	59.3	54.0	17.4	66.1	67.2	45.2	5.1
4 years	18 831	25.5	57.1	51.6	18.4	65.8	67.6	45.4	5.1
5 + years	22 748	25.3	49.9	44.9	18.9	66.0	67.3	45.6	4.9
Only child	88 481	24.7	40.3	38.2	22.4	65.2	68.1	46.6	4.6
Total	266 876	26.1	50.6	46.4	19.9	65.4	67.5	45.7	4.9