TABLE 2.

Percentages of Singleton Births That Were Preterm Births, by Selected Maternal Characteristics: United States, 1991, 1996, 2001, and 2006

Maternal Characteristic	1991 (n = 4 017 127), %	1996 (n = 3 788 111), %	2001 (n = 3 902 691), %	2006 (n = 4 129 440), %	% Change 1991–2006
Age, y
<20	13.9	12.9	13.3	13.9	0.0
20–34	9.1	9.1	9.8	10.5	15.4*
≥35	10.2	10.3	11.1	11.9	16.7*
Race/ethnicity
Hispanic	10.3	10.1	10.5	11.1	7.8*
Non-Hispanic White	7.7	8.1	9.0	9.7	26.0*
Non-Hispanic Black	17.9	16.3	16.0	16.6	−7.3*
Non-Hispanic American Indian	11.2	11.0	12.2	13.1	17.0*
Non-Hispanic Asian/Pacific Islander	9.5	9.2	9.2	9.5	0.0
Total birth order
1	9.8	9.8	10.3	10.8	10.2*
≥2	9.9	9.7	10.4	11.2	13.1*
Diabetes
Yes	12.0	13.4	14.3	15.1	25.8*
No	9.7	9.6	10.2	10.9	12.4*
Chronic hypertension
Yes	19.7	21.4	23.7	24.3	23.4*
No	9.7	9.6	10.3	10.9	12.4*
Pregnancy-associated hypertension
Yes	16.2	18.7	21.4	23.7	46.3*
No	9.5	9.4	10.0	10.6	11.6*

Note. Missing data were excluded from percentage computations. Data were derived from the natality detail files of the National Center for Health Statistics.

*P < .001.