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).