Table 3.

Adjusted odds ratios and 95% confidence intervals for categories of preterm birth, from the National Center for Health Statistics Natality Files 2004^*

	Spontaneous	Induced	Overall
Maternal Race/Ethnicity (Reference White N-H)
Black N-H	1.50 (1.48,1.52)	1.40 (1.38,1.41)	1.50 (1.49,1.51)
Hispanic	1.15 (1.13,1.17)	1.14 (1.12,1.15)	1.15 (1.14,1.16)
Asian & Pacific Islander	1.37 (1.34,1.41)	1.00 (0.97,1.03)	1.19 (1.18,1.20)
Native American	1.16 (1.12,1.21)	1.10 (1.06,1.15)	1.15 (1.14,1.16)
Maternal Age in Years (Reference 25-29)
< 15	2.01 (1.87,2.16)	1.01 (0.92,1.12)	1.60 (1.59,1.61)
15-19	1.30 (1.27,1.32)	0.75 (0.73,0.76)	1.02 (1.01,1.03)
20-24	1.08 (1.07,1.10)	0.85 (0.84,0.86)	0.96 (0.95,0.97)
30-34	0.96 (0.95,0.97)	1.22 (1.21,1.24)	1.09 (1.08,1.10)
35-39	0.96 (0.95,0.98)	1.50 (1.48,1.53)	1.24 (1.23,1.25)
40-45	0.99 (0.96,1.02)	1.97 (1.92,2.01)	1.49 (1.48,1.50)
45-49	0.85 (0.76,0.96)	2.42 (2.24,2.63)	1.67 (1.66,1.68)
Pregnancy Plurality (Reference Singleton)
Twin	2.94 (2.89,2.99)	15.5 (15.4,15.7)	13.1 (12.9,13.2)
Triplet+	0.99 (0.88,1.09)	158 (146,171)	142 (129,157)
Maternal Birth Place (Reference United States)
Canada	0.99 (0.90,1.09)	0.87 (0.79,0.95)	0.92 (0.91,0.93)
Mexico	0.89 (0.87,0.91)	0.70 (0.69,0.72)	0.78 (0.77,0.79)
Remainder of the World	0.91 (0.89,0.92)	0.83 (0.81,0.84)	0.85 (0.84,0.86)
Maternal Education (Reference 13-15 years)
0-8 years	1.34 (1.31,1.37)	1.02 (0.99,1.04)	1.20 (1.19,1.21)
9-11 years	1.26 (1.25,1.28)	1.14 (1.13,1.16)	1.23 (1.22,1.24)
12 years	1.11 (1.09,1.12)	1.06 (1.04,1.07)	1.08 (1.07,1.09)
16+ years	0.89 (0.88,0.91)	0.78 (0.77,0.79)	0.83 (0.82,0.84)

^*Adjusted also for marital status, gravidity and geographic division