Table 3. Association between clinical and demographic characteristics and risk for spontaneous preterm birth across maternal age groups.

Risk factor	< 16-year old	16–19.9-year old	≥ 20-year old
Race
Black	1.10 (0.88–1.37)	1.23 (1.16–1.31)	1.42 (1.38–1.47)
American Indian	0.68 (0.31–1.51)	1.11 (0.92–1.34)	1.30 (1.15–1.47)
Asian	2.10 (1.15–3.85)	1.45 (1.23–1.72)	1.04 (1.00–1.08)
Latina	0.98 (0.80–1.20)	1.07 (1.01–1.13)	1.13 (1.10–1.17)
White	REF	REF	REF
BMI
Underweight BMI	1.49 (1.73–1.90)	1.30 (1.21–1.39)	1.06 (1.00–1.11)
Overweight	0.96 (0.76–1.20)	1.08 (1.02–1.15)	1.24 (1.20–1.27)
Obese	0.80 (0.58–1.11)	0.94 (0.87–1.00)	1.14 (1.11–1.18)
Normal weight	REF	REF	REF
Recommended weight gain based on BMI
Under recommended	2.51 (2.01–3.02)	1.98 (1.88–2.09)	1.80 (1.76–1.85)
Over recommended	0.55 (0.43–0.69)	0.26 (0.53–0.59)	0.59 (0.57–0.60)
At recommended	REF	REF	REF
Kotelchuck's prenatal index
Inadequate	2.06 (1.62–2.63)	2.11 (1.97–2.26)	2.14 (2.07–2.22)
Adequate+	3.74 (2.90–4.81)	4.92 (4.61–5.24)	5.59 (5.43–5.75)
Adequate	REF	REF	REF
Infant sex
Male	1.27 (1.08–1.48)	1.20 (1.15–1.26)	1.25 (1.22–1.28)
Female	REF	REF	REF
Tobacco use	1.06 (0.70–1.61)	1.25 (1.17–1.34)	1.39 (1.34–1.45)
WIC assistance	0.55 (0.46–0.66)	0.71 (0.67–0.75)	0.86 (0.84–0.89)
Gonorrhea	1.71 (0.84–3.46)	1.23 (0.98–1.55)	1.13 (0.92–1.38)
Chlamydia	0.77 (0.54–1.09)	0.98 (0.89–1.08)	1.08 (1.00–1.17)

Abbreviations: BMI, body mass index; REF, referent; WIC, Women, Infants, and Children.

Note: data shown as relative risk with 95% confidence intervals.