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Table 2 Crude and adjusted odds ratios (with 95% confidence intervals in parentheses) for the prevalence of preterm birth according to birth year and maternal characteristics in California for the period 2007–2016

From: Temporal trends, patterns, and predictors of preterm birth in California from 2007 to 2016, based on the obstetric estimate of gestational age

Variable Crude odds ratio Adjusted odds ratio
OR (95% CI) p-valuea OR (95% CI) p-valuea
Birth year
 2008 0.99 (0.98–1.00) 0.148 0.99 (0.97–1.00) 0.110
 2009 0.95 (0.93–0.96) <.001 0.94 (0.92–0.96) <.001
 2010 0.93 (0.91–0.94) <.001 0.92 (0.91–0.94) <.001
 2011 0.92 (0.90–0.93) <.001 0.91 (0.90–0.93) <.001
 2012 0.92 (0.90–0.93) <.001 0.91 (0.90–0.93) <.001
 2013 0.91 (0.89–0.92) <.001 0.90 (0.89–0.92) <.001
 2014 0.89 (0.87–0.90) <.001 0.89 (0.87–0.90) <.001
 2015 0.92 (0.90–0.93) <.001 0.91 (0.89–0.93) <.001
 2016 0.94 (0.92–0.95) <.001 0.93 (0.91–0.94) <.001
 2007 Refb   Refb  
Maternal age (years)
  < 20 1.16 (1.15–1.18) <.001 1.07 (1.05–1.09) <.001
 25–29 0.97 (0.96–0.98) <.001 1.08 (1.07–1.10) <.001
 30–34 1.01 (1.00–1.02) 0.015 1.24 (1.22–1.26) <.001
 35–39 1.20 (1.19–1.22) <.001 1.51 (1.49–1.54) <.001
 40–54 1.54 (1.52–1.57) <.001 1.92 (1.88–1.96) <.001
 20–24 (ref) Refb   Refb  
Maternal race/ethnicity
 African American 1.77 (1.74–1.79) <.001 1.64 (1.61–1.67) <.001
 American Indian 1.46 (1.39–1.54) <.001 1.26 (1.18–1.35) <.001
 Asian 1.16 (1.14–1.17) <.001 1.40 (1.38–1.42) <.001
 Hispanic 1.23 (1.22–1.24) <.001 1.22 (1.21–1.24) <.001
 Multiple Race 1.26 (1.23–1.29) <.001 1.23 (1.20–1.26) <.001
 Other Unknown 1.35 (1.32–1.38) <.001 1.26 (1.22–1.30) <.001
 Pacific Islander 1.48 (1.41–1.55) <.001 1.43 (1.35–1.51) <.001
 White (ref) Refb   Refb  
Maternal education
  < High school 1.37 (1.36–1.38) <.001 1.35 (1.33–1.38) <.001
 High school diploma 1.28 (1.27–1.29) <.001 1.30 (1.28–1.31) <.001
 Some college/associate degree 1.27 (1.26–1.29) <.001 1.28 (1.26–1.30) <.001
 Unknown 1.39 (1.36–1.41) <.001 1.30 (1.27–1.34) <.001
 Bachelor’s degree or higher (ref) Refb   Refb  
Maternal nativity
 United States-born 1.08 (1.07–1.08) <.001 1.15 (1.14–1.16) <.0001
 Foreign-born (ref) Refb   Refb  
Maternal demographic region
 Central Coast 0.97 (0.95–1.00) 0.023 1.01 (0.98–1.04) 0.621
 Greater Bay Area 1.03 (1.00–1.05) 0.022 1.04 (1.01–1.06) 0.007
 Inland Empire 1.14 (1.11–1.16) <.001 1.13 (1.10–1.16) <.001
 Los Angeles County 1.13 (1.11–1.15) <.001 1.12 (1.09–1.14) <.001
 Orange County 0.99 (0.97–1.02) 0.493 1.05 (1.02–1.08) 0.001
 Sacramento Area 1.03 (1.01–1.06) 0.016 1.05 (1.02–1.08) 0.001
 San Diego Area 1.07 (1.04–1.09) <.001 1.13 (1.10–1.16) <.001
 San Joaquin Valley 1.21 (1.18–1.24) <.001 1.18 (1.15–1.22) <.001
 Northern and Sierra (ref) Refb   Refb  
Source of prenatal care payment
 Medi-Cal (Public) 1.13 (1.12–1.14) <.001 1.12 (1.11–1.13) <.001
 Private insurance (ref) Refb   Refb  
cWIC participation
 No 0.95 (0.94–0.96) <.001 1.13 (1.12–1.14) <.001
 Yes (ref) Refb   Refb  
First trimester prenatal care initiation
 No 1.08 (1.07–1.09) <.001 1.07 (1.06–1.09) <.001
 Yes (ref) Refb   Refb  
Parity
 Primiparous 1.03 (1.02–1.04) <.001 1.16 (1.15–1.17) <.001
 Multiparous 6–12 1.84 (1.80–1.88) <.001 1.35 (1.32–1.39) <.001
 Multiparous 2–5 (ref) Refb   Refb  
Maternal smoking
 Yes 1.85 (1.81–1.89) <.001 1.57 (1.53–1.62) <.001
 No (ref) Refb   Refb  
Maternal prepregnancy body mass index (kg/m2)
 Underweight, < 18.5 1.23 (1.20–1.25) <.001 1.23 (1.20–1.25) <.001
 Overweight, 25.0–29.9 1.09 (1.08–1.09) <.001 1.06 (1.05–1.07) <.001
 Obese I–30.0–34.9 1.22 (1.21–1.23) <.001 1.16 (1.15–1.17) <.001
 Obese II, 35.0–39.9 1.31 (1.29–1.33) <.001 1.24 (1.22–1.26) <.001
 Obese III, ≥ 40 1.44 (1.42–1.47) <.001 1.33 (1.30–1.36) <.001
 Normal, 18.5–24.9 (ref) Refb   Refb  
  1. OR Odds ratio, AOR Adjusted odds ratio, CI Confidence interval, BMI Body mass index
  2. ap value determined using the χ2 test
  3. bRef Reference group
  4. cWIC Supplemental Nutrition Program for Women, Infants, and Children (household incomes below 185% of the federal poverty line are eligible for WIC)
  5. All singleton live births were delivered at 17–47 completed weeks of gestation based on obstetric estimates and birthweight of ≥500 g at birth. Births with missing information on gestational age or birthweight were excluded
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