Holstius et al. Our analysis was limited by the availabil- ity of information on administrative birth records. Among the variables included on these forms, we adjusted for maternal charac- teristics (age, educational attainment, parity, race/ethnicity) and characteristics of the birth itself (infant’s sex, gestational age) known to have a substantial influence on birth weight. These variables may have confounding poten- tial, or play a role in explaining variation even in the absence of a confounding effect. We included parity of the mother with three levels: first live birth (reference), second live birth, or third or more live births. We coded gestational age (weeks since LMP) as 37 (ref- erence), 38, 39, 40, 41, or 42 weeks. As a proxy for socioeconomic status, we coded maternal education with four levels: less than a high school education; completed high school or equivalent (reference); 1–3 years of postsecondary education; and ≥ 4 years of postsecondary education. We coded maternal race/ethnicity as non-Hispanic white (refer- ence); Hispanic of any race; non-Hispanic black; non-Hispanic Asian; and unknown/ multiple/other. We also included fetal sex with male as the referent category. To account for secular trend and seasonal effects, we parameterized f (ti) as a combina- tion of a linear secular trend based on the date of the last menstrual period for birth i (ti) and categorical indicator terms for the season of birth (in quartiles): Q1 (January–March, refer- ence), Q2 (April–June), Q3 (July–September), or Q4 (October–December): f (ti) = βtrend (ti) + βQ2 IQ2 + βQ3 IQ3 + βQ4 IQ4 [2] Sensitivity analyses. In addition to our primary analyses, we conducted three sensitiv- ity analyses. In the first, we dichotomized the population according to whether the maternal residence census tract was closer to a PM10 monitor active during the fire period with an average PM10 measure > 40 µg/m3, or closer to a PM10 monitor with a lower average PM10 during the fire period. In the second, because of the association between season and trimes- ter of exposure (Table 1, Figure 2), we instead parameterized f (ti) as a smooth, periodic, sinu- soidal function of time known as the cosinor [Barnett and Dobson 2010; see Supplemental Material, Equations S1–S4 (http://dx.doi. Table 1. Maternal and infant characteristics (%), by wildfire event exposure status and trimester of exposure—SoCAB (n = 886,034). Exposed by trimester (n = 138,444) Unexposed (n = 747,590) First (n = 60,270) Second (n = 39,435) Third (n = 38,739) 50.9 49.1 3.9 9.7 21.9 29.5 24.1 10.9 39.5 31.3 29.1 3.0 76.4 20.5 31.9 28.6 17.9 21.5 60.5 21.3 11.9 6.0 0.3 87.6 12.4 0.0 0.0 org/10.1289/ehp.1104515)]. In the third, because accounts differ concerning the length of the wildfire event, we reduced the length of the wildfire event to peak exposure periods of 2 weeks or 1 week instead of 3 (keeping the starting date unchanged) and reassigned exposures accordingly. Statistical software. We used R version 2.14.0 and the stats::lm() function for model fitting. To fit the cosinor-based seasonal model, we used the season package, version 0.2–6 (R Project for Statistical Computing, Vienna, Austria). Results Of the 886,034 births in our analysis, 84.4% (n = 747,590) were unexposed in utero. Of the 138,444 exposed, 28.0% (n = 38,739) were exposed in the third trimester, 28.5% (n = 39,435) were exposed in the second tri- mester, and 43.5% (n = 60,270) were exposed in the first trimester (Table 1). Most infants in our study were delivered to Hispanic mothers (60.5%), followed by non-Hispanic white (22.0%), non-Hispanic Asian or Pacific Islander (11.3%), and non- Hispanic black (5.9%) mothers. Most infants were delivered to mothers 18–34 years of age (76.1%), with 20.8% delivered to mothers 35–50 years of age and 3.1% delivered to mothers 15–18 years of age. Approximately one-third of mothers had less than a high school education (32.1%), whereas 28.4% had completed a high school degree or equiv- alent, 18.1% had 1–3 years of postsecondary education, and 21.4% had ≥ 4 years of post- secondary education. We did not observe substantive differ- ences between the exposed and unexposed with respect to measured covariates, except for season of birth (Table 1). Effect estimates for covariates are reported in Supplemental Material, Table S1 (http://dx.doi.org/10.1289/ ehp.1104515). Estimated effects of wildfire exposure. Adjusted models revealed that mean birth weight was 6.1 g lower [95% confidence inter- val (CI): –8.7, –3.5] among infants exposed in utero during any trimester compared with unexposed infants [see Supplemental Material, Table S1 (http://dx.doi.org/10.1289/ ehp.1104515)]. Among those exposed in the third trimester, we observed a reduction of 7.0 g (95% CI: –11.8, –2.2). The largest esti- mated effect was observed in the second trimes- ter, with a reduction of 9.7 g (95% CI: –14.5, –4.8). Among infants exposed in the first tri- mester, we also estimated a decline in mean birth weight but it was not statistically signifi- cant (3.3 g; 95% CI: –7.2, 0.6) (Table 2). Both unadjusted and adjusted estimates are reported in Table 2; subsequent discussion is restricted to the adjusted model. We also restricted the peak exposure period by specifying models 120 | number 9 | September 2012 • Environmental Health Perspectives Gestational age (weeks) Variable Fetal sex Male Female 37 38 39 40 41 42 1 2 Parity < 18 18–34 35–50 3 or more Maternal age (years) Completed high school or equivalent Maternal education Less than high school 1–3 years postsecondary ≥ 4 years postsecondary Maternal race/ethnicity Hispanic Non-Hispanic white Non-Hispanic Asian Non-Hispanic black Non-Hispanic other/unknown Season Q1 (January–March) Q2 (April–June) Q3 (July–September) Q4 (October–December) 1342 51.0 49.0 4.8 10.9 23.0 29.2 22.6 9.4 38.1 31.8 30.1 3.1 76.1 20.8 32.3 28.4 18.1 21.2 60.6 22.0 11.2 5.9 0.3 24.8 23.0 23.0 29.3 51.0 49.0 4.7 10.6 22.6 29.9 22.6 9.6 38.7 31.8 29.5 3.0 75.8 21.2 31.0 27.6 18.5 22.9 60.0 22.6 11.4 5.7 0.3 0.0 0.0 66.7 33.3 50.7 49.3 5.0 11.1 22.9 29.5 21.9 9.6 38.8 31.5 29.7 3.0 76.0 21.0 31.7 27.8 17.8 22.7 59.9 22.3 11.6 5.9 0.3 0.0 94.8 5.2 0.0 ovlume