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. Author manuscript; available in PMC: 2020 May 1.
Published in final edited form as: Environ Int. 2019 Feb 15;126:7–13. doi: 10.1016/j.envint.2019.02.023

Table 1.

Demographics of preterm births among 403 US counties from 1989 to 2002.

Characteristics, No. (%) Preterm births (20–36 weeks) OR for preterm birth (95% CI)a
Total birth 2,973,909 (9.3)
Gestational age, weeks, median (IQR) 35.0 (3.0)
Maternal age, years
 <25 1,179,350 (10.7) 1.34 (1.33, 1.34)
 25–34 1,390,758 (8.3) 1 [Reference]
 ≥35 403,801 (9.9) 1.21 (1.21, 1.22)
Marital status
 Married 1,717,846 (7.8) 1 [Reference]
 Unmarried 1,256,063 (12.7) 1.71 (1.71, 1.72)
Maternal race
 White 1,999,007 (8.1) 1 [Reference]
 Nonwhite 974,902 (13.7) 1.81 (1.81, 1.82)
Years of education, years
 ≤ 8 227,284 (10.9) 1.04 (1.04, 1.04)
 9–12 1,571,182 (10.7) 1 [Reference]
 13–17 1,103,229 (7.7) 0.69 (0.68, 0.69)
 Unknown 72,214 (10.1) 0.95 (0.94, 0.95)
Smoking during pregnancy
 No 1,897,344 (9.1) 1 [Reference]
 Yes 344,353 (12.1) 1.36 (1.35, 1.36)
 Unknownb 732,212 (8.8) 0.96 (0.96, 0.96)
Alcohol consumption during pregnancy
 No 2,264,166 (9.4) 1 [Reference]
 Yes 51,698 (12.4) 1.37 (1.35, 1.38)
 Unknownb 658,045 (8.9) 0.94 (0.94, 0.94)
Parity
 0 1,261,636 (9.5) 1 [Reference]
 1 849,657 (8.3) 0.88 (0.87, 0.88)
 ≥ 2 850,362 (10.3) 1.11 (1.11, 1.11)
 Unknown 12,254 (10.7) 1.16 (1.14, 1.18)
Chronic hypertension
 No 2,865,650 (9.2) 1 [Reference]
 Yes 43,430 (20.9) 2.57 (2.54, 2.59)
 Unknown 64,829 (11.0) 1.23 (1.22, 1.24)
Season of conceptionc
 Spring 690,569 (9.4) 0.98 (0.98, 0.99)
 Summer 718,648 (9.3) 0.97 (0.97, 0.98)
 Fall 803,703 (9.5) 1 [Reference]
 Winter 760,989 (9.2) 0.96 (0.96, 0.96)
Geographic region
 Northeast 715,500 (8.8) 1.01 (1.01, 1.01)
 Southeast 566,639 (10.8) 1.26 (1.25, 1.26)
 Midwest 591,021 (9.5) 1.10 (1.09, 1.10)
 Great Plains 347,421 (10.1) 1.19 (1.18, 1.19)
 Northwest 74,808 (7.4) 0.84 (0.83, 0.84)
 Southwest 678,520 (8.7) 1 [Reference]
Climate zone
 Hot-Humid 533,446 (10.6) 1.26 (1.26, 1.27)
 Mixed-Humid 839,539 (10.0) 1.18 (1.18, 1.18)
 Hot-Dry/Mixed-Dry 528,042 (9.0) 1.06 (1.05, 1.06)
 Cold/Very Cold 914,837 (8.6) 1 [Reference]
 Marine 158,045 (7.5) 0.86 (0.86, 0.87)

Abbreviations: IQR, interquartile range; OR, odds ratio; CI, confidence interval.

denotes not applicable.

a

Odds ratios were estimated from crude model that without including any covariates.

b

Smoking and drinking consumption were not recorded on California birth certificates.

c

Season of conception: spring (March-May), summer (June-August), fall (September-November), and winter (December-February).