Table 2

Rate ratios (RR) of isolated congenital hydrocephalus (CHC), according to birth characteristics of the child among Danish children born 1978–2008

	Isolated CHC
	Children	Cases	RR*	95% CI	p Value†
Child's gender
Male	988 233	788	1.85	1.64 to 2.09
Female	937 251	405	1	(Ref.)
Multiples
Yes	60 304	94	2.84	2.29 to 3.51
No	1 865 180	1099	1	(Ref.)
Gestational age (weeks)
<28	4424	29	24.7	16.8 to 36.2	<0.0001
28–31	11 618	74	13.6	10.6 to 17.4
32–36	84 426	178	4.07	3.43 to 4.82
37–41	1 562 531	777	1	(Ref.)
≥42	198 090	84	0.86	0.69 to 1.08
Missing‡	64 395	51	1.18	0.88 to 1.58
Weight for gestational age (percentiles)
<2.5	44 588	79	2.78	2.19 to 3.53	<0.0001
2.5–4	42 515	37	1.45	1.04 to 2.02
5–9	85 574	84	1.71	1.36 to 2.15
10–14	87 498	57	1.03	0.78 to 1.36
15–19	101 783	70	1.22	0.95 to 1.56
20–79	1 110 815	586	1	(Ref.)
80–84	99 255	55	1.12	0.85 to 1.47
85–89	85 850	38	0.91	0.65 to 1.26
90–94	99 976	67	1.40	1.09 to 1.81
95–97.5	46 466	27	1.23	0.84 to 1.81
>97.5	48 828	35	1.56	1.10 to 2.19
Missing‡	72 336	58	1.21	0.92 to 1.60
Mode of delivery, differentiated§
Caesarean section (CS), acute	24 089	46	4.30	3.11 to 5.95	<0.0001
CS, planned	53 634	58	2.65	1.98 to 3.56
CS during birth, not acute	8349	16	4.06	2.42 to 6.79
CS during birth, acute	60 486	60	2.15	1.61 to 2.88
Vaginal delivery	638 413	249	1	(Ref.)

*Adjusted for year of birth, maternal age, parity, child's gender and multiple births.
†Test for homogeneity.
‡Individuals with missing information were not included in the tests.
§Information on mode of delivery was available from 1997 and onward.