Table 2.

Socio-demographic information about parents

Perthes’	No		Yes
	n	%	n	%	p-value
Educational level, father
< 11	493,802	23	566	29	< 0.001
11–13	990,041	47	929	47
≥ 14	580,572	27	445	22
Missing	54,803	3	34	2
Educational level, mother
< 11	321,002	15	392	20	< 0.001
11–13	1,040,430	49	1,038	53
≥ 14	721,578	34	528	27
Missing	35,208	2	16	1
Origin
Both Nordic	1,903,452	90	1,846	94	< 0.001
One or both non-Nordic	214,766	10	128	6
Marital status, mother
Married	679,764	32	627	32	0.4
Unmarried	1,298,018	66	1,301	66
Widowed/divorced	40,322	2	46	2
Mother’s age when she gave birth
13–19	88,965	4	96	5	0.1
20–26	868,816	41	842	43
27–33	890,507	42	785	40
> 33	269,930	13	251	13
Parity
Primi	880,212	42	771	39	0.02
Multi	1,238,006	58	1,203	61
Birth order
1	877,094	41	765	39	0.3
2	777,687	37	757	38
3	331,720	16	311	16
4	93,632	4	89	4
5	25,114	1.2	32	1.6
6	7756	0.4	14	0.7
7	2848	0.1	2	0.1
8	1128	0.1	3	0.2
9	661	0	1	0.1
10	297	0	0	0
11	140	0	0	0
12	73	0	0	0
13	34	0	0	0
14	14	0	0	0
15	12	0	0	0
16	4	0	0	0
17	3	0	0	0
18	1	0	0	0
Number of siblings
1	941,347	23	513	26	0.06
2	941,983	44	869	44
3	496,672	23	440	22
4	139,867	7	119	6
5	34,008	2	24	1
6	9735	0.5	6	0.3
7	2937	0.1	3	0.2
8 or more	1669	0.1	0	0.0
Annual income, mother
≤ 10th percentile	209,921	10	174	9	0.2
> 10th and <90th	1,623,177	78	1,545	79.0
≥ 90th percentile	248,093	12	237	12
Annual income, father
≤ 10th percentile	207,953	10	192	10	0.03
> 10th and <90th	1,657,300	80	1596	82
≥ 90th percentile	206,044	10	159	8
Annual income, family
≤ 10th percentile	207,683	10	182	9	0.08
> 10th and <90th	1,643,196	79	1,584	81
≥ 90th percentile	218,143	11	180	9
Smokinga
No	43,610	71	34	49	< 0.001
Yes	17,550	29	35	51

^aOnly available for the years 1990–1993.