Table 3.
Characteristics of the Study Population (n = 237,031) by Quartile of Traffic Density, Study of Air Pollution, Genetics and Early Life Events, San Joaquin Valley of California, 2000–2006
| Covariate | First Quartile (n = 59,197) |
Second Quartile (n = 59,271) |
Third Quartile (n = 59,210) |
Fourth Quartile (n = 59,353) |
||||
|---|---|---|---|---|---|---|---|---|
| No. | % | No. | % | No. | % | No. | % | |
| Birth weight | ||||||||
| Low (<2,500 g) | 1,149 | 1.9 | 1,322 | 2.2 | 1,232 | 2.1 | 1,420 | 2.4 |
| Normal (≥2,500 g) | 58,048 | 98.1 | 57,949 | 97.8 | 57,978 | 97.9 | 57,933 | 97.6 |
| Maternal age, years | ||||||||
| <20 | 6,878 | 11.6 | 7,845 | 13.2 | 8,287 | 14.0 | 9,260 | 15.6 |
| 20–35 | 45,014 | 76.1 | 44,706 | 75.5 | 45,015 | 76.0 | 45,084 | 76.0 |
| >35 | 7,305 | 12.3 | 6,720 | 11.3 | 5,908 | 10.0 | 5,009 | 8.4 |
| Maternal race/ethnicity | ||||||||
| Asian | 4,582 | 7.7 | 4,135 | 7.0 | 4,006 | 6.8 | 5,012 | 8.4 |
| African-American | 1,826 | 3.1 | 2,511 | 4.2 | 3,003 | 5.1 | 4,220 | 7.1 |
| Hispanic | 32,054 | 54.2 | 32,880 | 55.5 | 33,892 | 57.2 | 33,779 | 56.9 |
| White | 19,631 | 33.2 | 18,837 | 31.8 | 17,553 | 29.7 | 15,501 | 26.1 |
| Other | 1,104 | 1.9 | 908 | 1.5 | 756 | 1.3 | 841 | 1.4 |
| Maternal education | ||||||||
| No high school | 6,915 | 11.7 | 6,743 | 11.4 | 7,360 | 12.4 | 7,009 | 11.8 |
| Some high school | 28,352 | 47.9 | 29,826 | 50.3 | 31,722 | 53.6 | 34,228 | 57.7 |
| Some college | 12,898 | 21.8 | 12,670 | 21.4 | 11,989 | 20.3 | 11,855 | 20.0 |
| Bachelor's or other degree | 9,795 | 16.6 | 8,704 | 14.7 | 6,663 | 11.3 | 4,928 | 8.3 |
| Missing data | 1,237 | 2.1 | 1,328 | 2.2 | 1,476 | 2.5 | 1,333 | 2.2 |
| Birth costs paid by Medi-Cal | ||||||||
| Yes | 27,547 | 46.5 | 30,053 | 50.7 | 33,024 | 55.8 | 36,940 | 62.2 |
| No | 31,650 | 53.5 | 29,218 | 49.3 | 26,186 | 44.2 | 22,413 | 37.8 |
| Low socioeconomic statusa | ||||||||
| Yes | 5,654 | 9.6 | 9,065 | 15.3 | 11,084 | 19.8 | 15,942 | 26.9 |
| No | 53,543 | 90.5 | 50,206 | 84.7 | 48,126 | 81.3 | 43,411 | 73.1 |
| Parity | ||||||||
| 0 | 20,468 | 34.6 | 20,910 | 35.3 | 20,809 | 35.1 | 21,632 | 36.4 |
| ≥1 | 38,729 | 65.4 | 38,361 | 64.7 | 38,401 | 64.9 | 37,721 | 63.6 |
| Sex of infant | ||||||||
| Male | 30,129 | 50.9 | 29,948 | 50.5 | 30,059 | 50.8 | 30,320 | 51.1 |
| Female | 29,068 | 49.1 | 29,323 | 49.5 | 29,151 | 49.2 | 29,033 | 48.9 |
| Initiation of prenatal care | ||||||||
| First trimester | 48,781 | 82.4 | 49,195 | 83.0 | 47,456 | 80.2 | 47,473 | 80.0 |
| Second trimester | 7,627 | 12.9 | 7,428 | 12.5 | 8,618 | 14.6 | 9,003 | 15.2 |
| Third trimester | 1,823 | 3.1 | 1,596 | 2.7 | 1,968 | 3.3 | 1,930 | 3.2 |
| Unknown | 966 | 1.6 | 1,052 | 1.8 | 1,168 | 2.0 | 947 | 1.6 |
| Year of birth | ||||||||
| 2000 | 7,194 | 12.2 | 7,766 | 13.1 | 7,879 | 13.3 | 7,949 | 13.4 |
| 2001 | 7,470 | 12.6 | 7,862 | 13.3 | 8,127 | 13.7 | 8,248 | 13.9 |
| 2002 | 7,762 | 13.1 | 8,208 | 13.9 | 8,276 | 14.0 | 8,288 | 14.0 |
| 2003 | 8,183 | 13.8 | 8,260 | 13.9 | 8,380 | 14.2 | 8,259 | 14.0 |
| 2004 | 8,762 | 14.8 | 8,597 | 14.5 | 8,489 | 14.3 | 8,483 | 14.3 |
| 2005 | 9,339 | 15.8 | 8,951 | 15.1 | 8,560 | 14.5 | 8,717 | 14.7 |
| 2006 | 10,487 | 17.7 | 9,627 | 16.2 | 9,499 | 16.0 | 9,409 | 15.8 |
| County of maternal residence | ||||||||
| Fresno | 13,910 | 23.5 | 21,155 | 35.7 | 16,303 | 27.5 | 25,725 | 43.3 |
| Kern | 15,698 | 26.5 | 16,488 | 27.8 | 14,574 | 24.6 | 9,558 | 16.1 |
| San Joaquin | 16,456 | 27.8 | 11,023 | 18.6 | 15,787 | 26.7 | 16,414 | 27.7 |
| Stanislaus | 13,133 | 22.2 | 10,605 | 17.9 | 12,546 | 21.2 | 7,656 | 12.9 |
a Low socioeconomic status was defined as unemployment greater than 10%, more than 15% of households receiving public assistance, and more than 20% of families living below the federal poverty line at the block-group level in the 2000 US Census.