observational studies of chronic exposures. With this in mind, we designed a time-series study to observe trimester-specific differences in mean birth weight among infants delivered to mothers residing in the South Coast Air Basin (SoCAB) before, during, and after the Southern California wildfires of 2003. Methods Study population. We obtained birth records for infants delivered in the SoCAB from 1 January 2001 through 31 December 2005 from the Non-Confidential Birth Statistical Master File, provided by California’s Center for Health Statistics at the California Department of Health Services (California Automated Vital Statistics System 2006, unpublished data). We excluded preterm births (< 37 weeks gestation), post-term births (> 42 weeks gestation), and births with a reported birth weight < 1 kg or > 6 kg, yielding a total of 886,034 births for our analysis. Gestational ages were based on the number of days since the mother’s reported last menstrual period (LMP). The SoCAB, which forms the geographic basis for our study population, includes the entirety of Orange County as well as populous areas within Los Angeles, San Bernardino, and Riverside counties. This air basin was chosen as the boundary for the study on the basis of satellite images of the fires; it is partially bounded by mountains that trap air pollu- tion in the absence of wind (Figure 1). The SoCAB does not contain Ventura and San Diego counties, which were also exposed to smoke during the 2003 wildfires. Exposure assessment. On the basis of reports from the California Department of Forestry and Fire Protection (Blackwell and Tuttle 2003) and inspection of Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery (NASA 2011), we defined the window of potential wildfire expo- sure as 21 October–10 November 2003. Most births in our study population (n = 491,496) were delivered before 21 October 2003 and could not have been exposed in utero. An addi- tional 256,094 births were assigned an LMP later than 10 November 2003, and were also classified as unexposed. All remaining births (n = 138,444) were classified as exposed on the basis of temporal overlap between the wildfire exposure window and gestational intervals. Our primary analysis used this temporal contrast as the basis for exposure assessment. However, we also conducted a sensitivity analysis in which we examined spatial con- trasts based on proximity of maternal residence census tracts to air monitors. Tracts closer to monitors with average PM10 measures during the fires of < 40 µg/m3 were classified as low exposure, and tracts with average daily levels > 40 µg/m3 were classified as high exposure. This cut point split the PM10 monitors in the SoCAB in half, with 36% of births that ges- tated during the fires occurring in high expo- sure census tracts. Covariates and primary model. We fit the data to a linear fixed-effects model (Equation 1) with birth weight (yi) as a continuous outcome and xij as an indicator of exposure for birth i in trimester j. For our primary analysis, we defined xi as a categorical variable with four levels: exposed in trimester 1; exposed in trimester 2; exposed in trimester 3; or unexposed (Figure 2). We defined the first trimester as weeks 1–16 Reduced birth weight following wildfire since LMP; the second as weeks 17–28; and the third as week 29 through the end of gestation. When the wildfire event overlapped with two trimesters, we assigned exposure to the trimester with the greater number of days of overlap. We controlled for maternal and birth characteristics (zi) associated with birth weight and included terms based on the date of the LMP (ti) to control for seasonality and trend. yi = β0 + β1xi1 + β2xi2 + β3xi3 + η´zi + f (ti) + εi [1] South coast air basin County boundaries Figure 1. Geographic extent of the SoCAB study area, outlined in yellow, overlaid on MODIS satellite image from 26 October 2003. Active fires were outlined in red by NASA (NASA 2011). Wildﬁre event January 2003 April 2003 July 2003 October 2003 January 2004 April 2004 July 2004 October 2004 Figure 2. Schematic illustrating exposure assignment. Exposure status was assigned based on the over- lap between the wildfire event (yellow) and estimated gestational intervals (horizontal segments). For clarity, gestational intervals are shown ordered from top to bottom by the LMP, and only a 0.1% sample from 2002–2004 is shown. Dates on the x-axis correspond to the beginning of quarters used to adjust for seasonality. Environmental Health Perspectives • volume 120 | number 9 | September 2012 1341