Table 5.

Crude and adjusted odds ratios of preterm birth^a associated with mothers’ socio demographic characteristics

Periods		1991-1995				1996-2000				2001-2008
Mothers’ characteristicsb		Crude OR	CI [95%]	Adjusted ORc	CI [95%]c	Crude OR	CI [95%]	Adjusted OR	CI [95%]	Crude OR	CI [95%]	Adjusted OR	CI [95%]
Age	<18	1.42	[1.38,1.47]	1.26	[1.22,1.30]	1.33	[1.29,1.37]	1.24	[1.21,1.28]	1.28	[1.25,1.31]	1.20	[1.17,1.23]
	18-38		1.0 Referenced				1.0 Reference				1.0 Reference
	>38	1.76	[1.69,1.82]	1.78	[1.72,1.85]	1.76	[1.70,1.82]	1.82	[1.75,1.88]	1.59	[1.56,1.63]	1.65	[1.61,1.69]
Marital Status	Living with a partner		1.0 Reference				1.0 Reference				1.0 Reference
	Not living with a partner	1.20	[1.18,1.22]	1.16	[1.14,1.18]	1.06	[1.04,1.08]	1.02	[1.00,1.04]	1.03	[1.02,1.04]	1.17	[1.14,1.20]
Parity	Primiparous	1.09	[1.08,1.12]	1.07	[1.05,1.09]	1.10	[1.08,1.12]	1.11	[1.09,1.13]	1.10	[1.09,1.12]	1.12	[1.11,1.14]
	Multipara		1.0 Reference				1.0 Reference				1.0 Reference
	Grand multipara	1.38	[1.31,1.46]	1.33	[1.25,1.42]	1.30	[1.22,1.38]	1.28	[1.19,1.38]	1.30	[1.23,1.37]	1.26	[1.18,1.35]

Chile, Singletons live births, 1991–2008.

^aPreterm births : < 37 weeks of gestational age.

^bReference classes: 18–38 years, living with partner, multipara.

^cAll adjusted OR ( for the others factors in table) with P < 0.001 for Wald’s chi-square.