Table 4. Comparison of linked and unlinked baby record characteristics for 2012/13.

Missing values are excluded from all categories.

		Linked N = 660,401	% 98%	Unlinked N = 12,654	% 2%	p-value
Missing all baby tail fields		62157	9.4	7452	58.9	<0.001
Maternal age (years)	Mean	29.3		29.6		0.003
	< = 18	11452	2.6	103	2.9
	19–24	87116	20.0	793	22.0
	25–29	120943	27.7	1036	28.8
	30–34	129398	29.7	898	24.9
	35–39	68916	15.8	519	14.4
	> = 40	18008	4.1	252	7.0
IMD quintile*	Most deprived	9269	19.7	1107	22.2	<0.001
	2	9410	20.0	970	19.5
	3	9388	20.0	1000	20.1
	4	9380	20.0	1012	20.3
	Least deprived	9491	20.2	894	17.9
Ethnicity	White	457181	75.2	7855	71.0	<0.001
	Mixed	29234	4.8	693	6.3
	Asian	68588	11.3	1219	11.0
	Black	32410	5.3	735	6.6
	Other	20299	3.3	567	5.1
Gestational age (weeks)	Median	40		39		<0.001
	< = 27	5674	1.0	174	4.4
	28-<32	6591	1.2	84	2.1
	32-<37	31872	5.8	348	8.8
	37-<42	478042	87.5	3229	81.3
	> = 42	23881	4.4	137	3.4
Birth weight (g)	Mean	3342		3106		<0.001
	<1000	2837	0.5	159	3.4
	1000–1499	3516	0.6	116	2.5
	1500–2499	33021	5.8	498	10.6
	2500–3999	465984	81.7	3513	74.6
	> = 4000	64710	11.4	425	9.0
First antenatal assessment <20 weeks		443262	88.8	2508	86.7	<0.001
Caesarean section		146077	24.8	787	17.1	<0.001
Still birth		2752	0.4	285	2.3	<0.001
Multiple birth		20436	3.1	352	2.8	0.041

*IMD = Index of Multiple Deprivation