Table 1.
Description of the study population and environmental conditions, by gestational age category in California, May-September 2005 – 2013.
| Characteristics | Preterm birth (< 37 weeks) n = 127,039 | Term birth (≥ 37 weeks) n = 1,763,247 | Total births* n = 1,967,300 |
|---|---|---|---|
|
| |||
| Maternal age, years, mean (SD) | 28.36 (6.7) | 28.29 (6.2) | 28.30 (6.3) |
| Maternal race/ethnicity, n (%) | |||
| American Indian/Alaskan Native | 667 (0.5) | 7582 (0.4) | 8557 (0.4) |
| Asian | 14630 (11.5) | 220431 (12.5) | 244711 (12.4) |
| Hispanic | 69499 (54.7) | 896736 (50.9) | 1005799 (51.1) |
| Native Hawaiian/Pacific Islander | 804 (0.6) | 8918 (0.5) | 10138 (0.5) |
| Non-Hispanic Black | 10723 (8.4) | 94844 (5.4) | 109931 (5.6) |
| Non-Hispanic White | 28635 (22.5) | 505522 (28.7) | 555511 (28.2) |
| Maternal education, n (%) | |||
| ≤ 8th grade | 37341 (30.5) | 425993 (25.0) | 482210 (25.4) |
| 9th grade – high school | 33139 (27.1) | 435511 (25.6) | 487902 (25.7) |
| At least come college | 51979 (42.5) | 841046 (49.4) | 929120 (48.9) |
| Health insurance, n (%) | |||
| Medicaid | 66787 (52.8) | 815952 (46.4) | 918890 (46.8) |
| Private health insurance | 51910 (41.0) | 829836 (47.1) | 917391 (46.7) |
| Other | 7847 (6.2) | 114598 (6.5) | 127523 (6.5) |
| Parity, n (%) | |||
| 1 | 48634 (38.6) | 699535 (39.7) | 779126 (39.7) |
| 2 | 35146 (27.9) | 561898 (31.9) | 620926 (31.6) |
| 3 | 22001 (17.5) | 299279 (17.0) | 334441 (17.0) |
| 4 or more | 20093 (16.0) | 201457 (11.4) | 230483 (11.7) |
| Sex of infant, n (%) | |||
| Female | 56891 (44.8) | 863502 (49.0) | 958057 (48.7) |
| Male | 70139 (55.2) | 899741 (51.0) | 1009228 (51.3) |
| Season of birth, n (%) | |||
| Spring (May - June) | 50647 (39.9) | 668504 (37.9) | 719151 (36.6) |
| Summer (July - September) | 76392 (60.1) | 1094743 (62.1) | 1171135 (59.5) |
| Fall (October) | NA | NA | 77014 (3.9) |
| Environmental condition during the week preceding birth, mean (SD) | |||
| Temperature, °F | 83.52 (9.9) | 83.00 (9.8) | 83.12 (9.8) |
| PM2.5, μg/m3 | 11.48 (4.7) | 11.19 (4.6) | 11.26 (4.7) |
| PM10, μg/m3 | 30.34 (12.6) | 29.48 (12.3) | 29.69 (12.4) |
| O3, ppb | 50.36 (12.7) | 49.37 (12.5) | 49.39 (12.5) |
| NO2, ppb | 12.68 (6.2) | 12.27 (6.1) | 12.58 (6.4) |
| NDVI | 0.37 (0.1) | 0.37 (0.1) | 0.37 (0.1) |
SD, standard deviation; F, Fahrenheit.; ppb, parts per billion.
*
Birth outcomes are censored if the mother was at-risk during the study period (May to September) but gave birth outside this window. This study included 1,890,286 births and 77,014 fetus-at-risk.