Table 1.
Characteristics of the Study Population (n = 237,031), Overall and by Birth Weight, Study of Air Pollution, Genetics and Early Life Events, San Joaquin Valley of California, 2000–2006
| Covariate | Total Study Population (n = 237,031) |
Low Birth Weight Infants (n = 5,123; 2.2%) |
Normal Birth Weight Infants (n = 231,908; 97.8%) |
|||
|---|---|---|---|---|---|---|
| No. | % | No. | % | No. | % | |
| Maternal age, years | ||||||
| <20 | 32,270 | 13.6 | 968 | 18.9 | 31,302 | 13.5 |
| 20–35 | 179,819 | 75.9 | 3,561 | 69.5 | 176,258 | 76.0 |
| >35 | 24,942 | 10.5 | 594 | 11.6 | 24,348 | 10.5 |
| Maternal race/ethnicity | ||||||
| Asian | 17,738 | 7.5 | 584 | 11.4 | 17,154 | 7.4 |
| African-American | 11,560 | 4.9 | 527 | 10.3 | 11,033 | 4.8 |
| Hispanic | 132,605 | 55.9 | 2,653 | 518 | 129,952 | 56.0 |
| White | 71,522 | 30.2 | 1,261 | 24.6 | 70,261 | 30.3 |
| Other | 3,606 | 1.5 | 98 | 1.9 | 3,508 | 1.5 |
| Maternal education | ||||||
| No high school | 28,027 | 11.8 | 539 | 10.5 | 27,488 | 11.9 |
| Some high school | 124,128 | 52.4 | 3,021 | 59.0 | 121,107 | 52.2 |
| Some college | 49,412 | 20.8 | 975 | 19.0 | 48,437 | 20.9 |
| Bachelor's or other degree | 30,090 | 12.7 | 452 | 8.8 | 29,638 | 12.8 |
| Missing data | 5,374 | 2.3 | 136 | 2.7 | 5,238 | 2.3 |
| Birth costs paid by Medi-Cal | ||||||
| Yes | 127,564 | 53.8 | 3,110 | 60.7 | 124,454 | 53.7 |
| No | 109,467 | 46.2 | 2,013 | 39.3 | 107,454 | 46.3 |
| Low socioeconomic statusa | ||||||
| Yes | 41,745 | 17.6 | 1,102 | 21.5 | 40,643 | 17.5 |
| No | 195,286 | 82.4 | 4,021 | 78.5 | 191,265 | 82.5 |
| Parity | ||||||
| 0 | 83,819 | 35.4 | 2,303 | 45.0 | 81,516 | 35.1 |
| ≥1 | 153,212 | 64.6 | 2,820 | 55.0 | 150,392 | 64.9 |
| Sex of infant | ||||||
| Male | 120,456 | 50.8 | 2,221 | 43.4 | 118,235 | 51.0 |
| Female | 116,575 | 49.2 | 2,902 | 56.6 | 113,673 | 49.0 |
| Initiation of prenatal care | ||||||
| First trimester | 192,905 | 81.4 | 3,887 | 75.9 | 189,018 | 81.5 |
| Second trimester | 32,676 | 13.8 | 870 | 17.0 | 31,806 | 13.7 |
| Third trimester | 7,317 | 3.1 | 193 | 3.8 | 7,124 | 3.1 |
| Unknown | 4,133 | 1.7 | 173 | 3.4 | 3,960 | 1.7 |
| Year of birth | ||||||
| 2000 | 30,788 | 13.0 | 608 | 11.9 | 30,180 | 13.0 |
| 2001 | 31,707 | 13.4 | 675 | 13.2 | 31,032 | 13.4 |
| 2002 | 32,534 | 13.7 | 688 | 13.4 | 31,846 | 13.7 |
| 2003 | 33,082 | 14.0 | 717 | 14.0 | 32,365 | 14.0 |
| 2004 | 34,331 | 14.5 | 700 | 13.7 | 33,631 | 14.5 |
| 2005 | 35,567 | 15.0 | 816 | 15.9 | 34,751 | 15.0 |
| 2006 | 39,022 | 16.5 | 919 | 17.9 | 38,103 | 16.4 |
| County of maternal residence | ||||||
| Fresno | 77,093 | 32.5 | 1,757 | 34.3 | 75,336 | 32.5 |
| Kern | 56,318 | 23.8 | 1,253 | 24.5 | 55,065 | 23.7 |
| San Joaquin | 59,680 | 25.2 | 1,293 | 25.2 | 58,387 | 25.2 |
| Stanislaus | 43,940 | 18.5 | 820 | 16.0 | 43,120 | 18.6 |
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.