Table 6.

Crude and adjusted or for preterm births^a by period

Period	1991-1995	1996-2000		2000-2008
ORb		OR	CI 95%	OR	CI 95%
Crude OR	Reference	1.07	[1.06, 1.08]	1.30	[1.28,1.31]
		OR	CI 95%	OR	CI 95%
Ajusted OR	Reference	1.06	[1.05, 1.07]	1.26	[1.25,1.28]

Chile, Singletons live births, 1991–2008.

^aPreterm births : < 37 weeks of gestational age.

^bAll crude and adjusted OR ( adjusted for age, parity and marital status) with P < 0.001 for Wald’s chi-square.