approximately 6 weeks after a live delivery (postnatal); and from 10 women after 12 unsuccessful menstrual cycles without preg- nancy (infertile). Thus, we have a blood speci- men reflecting the varying critical windows (preconception, postimplantation, and postna- tal) and reproductive outcomes (pregnancy loss, live birth, infertility). The blood samples were transported on ice to the toxicology lab- oratory; these included 73 (92%) baseline or preconception, 53 (98%) prenatal, 10 (100%) EPL, 3 (75%) CPL, 52 (96%) postnatal, and 9 (90%) infertility blood samples. Toxicologic analysis. Serum samples were mixed with solutions of International Union of Pure and Applied Chemistry PCB isomers no. 46 and 142 (surrogate standards) and left overnight to equilibrate (Greizerstein et al. 1997). Methanol was added to precipitate the proteins, and the resulting mixture was extracted with hexane in a rotating extraction device at 50 rpm for 20 hr. Samples were then centrifuged and the extract was concen- trated under a slow stream of nitrogen at 50° C to 2 mL. The extract was placed on a deactivated Florisil column and eluted with hexane. The eluate was evaporated to a small volume under a slow stream on nitrogen using 200 µL iso-octane as a keep solvent. Isomers no. 30 and 204 were added as internal stan- dards to each extract. An aliquot of the mix- ture was injected into the gas chromatograph equipped with an electron capture detector (Agilent Technologies, Santa Clara, CA). A quality control (QC) sample was made from a matrix blank consisting of sheep serum, which was spiked with 0.6 ng/g PCB congeners 6, 44, and 52; 0.3 ng congeners 101, 138, 153, 180, 185, and 205; and 0.3 ng pesticides dichlorodiphenyldichloroethylene, hexachlorobenzene, and mirex. The QC sam- ple was run with each batch of 10 samples and appropriate reagent and matrix blanks. Quality control charts were kept throughout the study and any batch of samples in which the QC values exceeded acceptance criteria were rerun. Surrogate congener standards 46 and 142 were added to each sample to assess recoveries. The surrogate standard recoveries for one set of 500 serum samples was 85.5 ± 18.6% for PCB-46 and 83.1 ± 16.4% for PCB-142. Serum specimens were run in batches of 10 plus four quality control sam- ples: reagent blank, matrix blank, matrix blank containing a mixed standard of 15 specific congeners and pesticide components at known values, and a duplicate participant sample. The laboratory is in compliance in the AMAP Ring Test Proﬁciency Program for Persistent Organic Pollutants in Human Serum (Centre de toxicologie Institut national de santé publique du Québec, Québec, Canada). We corrected laboratory observed values only for recovery to minimize measurement error and potential biases associated with sub- stitution patterns below the limits of detec- tion (LOD) (Richardson and Ciampi 2003; Schisterman et al. 2006). A priori, 64 single- eluting and 12 di-eluting congeners were quantiﬁed and summed into three groupings (Cooke et al. 2001): a) total PCB congeners; b) estrogenic congeners (4 + 10, 8 + 5, 15 + 17, 18, 31, 44, 47, 48, 52, 70, 77 + 110, 99, 101, 126, 136, 153, and 188); and c) anti- estrogenic congeners (77 + 110, 105, 114, 118, 126, 156 + 171, and 169). We assessed PCB congener 153 individually for compari- son with other work focusing on female fecundity (Axmon et al. 2001). We also assessed PCB-118 individually given that most concentrations were above the LOD, similar to PCB-153, and to aid in the inter- pretation of results by PCB grouping. We quantiﬁed total serum lipids (TL) using enzy- matic methods as the function of total choles- terol (TC) and triglycerides (TG) expressed in milligrams per deciliter. We assumed free cholesterol to be 27% of the total, and pre- dicted phospholipids from regression on total cholesterol where TL = 2.27 TC + TG + 0.623 (Phillips et al. 1989). For analysis pur- poses, PCB concentrations are reported as nanograms per gram wet weight with serum lipids entered as a covariate in the analytic model to minimize bias arising from auto- matic lipid adjustment of PCB concentrations (Schisterman et al. 2005). Statistical analysis. With descriptive statis- tics, many PCB congener distributions were not normally distributed even after Box-Cox transformations; therefore, no further transfor- mations were undertaken. We used bivariate analysis to explore associations between the changes in concentrations from the baseline or preconception to the next serum measurement in relation to serum lipids (milligrams per deciliter) and relevant study covariates—i.e., body mass index (weight in kilograms/height in square meters), gravidity (number of preg- nancies), parity (number of births), and life- time duration of lactation in months (Glynn et al. 2003; Grimvall et al. 1997; Kostyniak et al. 1999; Sweeney et al. 2001). We estimated Spearman rank correlation coefficients to describe correlations between baseline PCB groupings, and between the measurements across critical windows. For each correlation, we used the chi-square test to obtain the level of signiﬁcance against the (zero correlation), using Fisher’s z-transforma- tion [z = 0.5 log(1 + r)/(1 – r)] with the appropriate variance correction. We estimated the overall amount of change between measurements and the daily rate of change in serum PCB concentrations and total serum lipids for 48 paired pre- conception-prenatal specimens, 47 paired prenatal–postnatal specimens, 10 paired Environmental Health Perspectives • VOLUME 115 | NUMBER 9 | September 2007 PCBs and critical windows preconception–EPL specimens, and 9 paired preconception–infertility specimens, using the following algorithm: Dijk = Yijk −Yij′k , [1] where Yijk represents measures for the ith (1,……,n) participant; for the jth PCB group- ing (where 1 = total PCBs, 2 = estrogenic PCBs, 3 = antiestrogenic PCBs, 4 = PCB-118, 5 = PCB-153, and 6 = total serum lipids vari- able, respectively); demonstrating the kth (1, 2, 3, 4) reproductive outcome (where 1 = pre- natal or a pregnancy resulting in a live birth, 2 = EPL or a pregnancy resulting in an early loss, 3 = infertile or 12 menstrual cycles with- out conception, and 4 = postnatal after a live birth; and Yi´jk represents the baseline or pre- conception measurement for k = 1, 2, or 3, and the prenatal measurement from a preg- nancy resulting in a live birth where k = 4. Women’s daily rate of change, deﬁned as the difference in the concentration between the ﬁrst (baseline) and second specimen collec- tion divided by the number of days between specimen collections, was derived as follows: Rijk = Dijk ti , [2] where ti represents the duration in days, for the ith (1,…,n) participant, between the reported positive hCG pregnancy test date and the out- come specimen sample date, where k = 1, 2, between the dates of the baseline and outcome specimens where k = 3, and the date of the pre- natal and postnatal specimens where k = 4. We assumed no change in PCB concentration between the date of baseline measure and the date of the positive hCG test among those women who conceived. We further estimated the daily rates of change in concentrations from the time of the positive pregnancy test to the second blood specimen. Length of gesta- tion at the time of the prenatal sample ranged from 8 to 126 days with a mean (± SD) of 31.4 ± 17.5 days and a median of 29 days. We estimated the mean overall and daily rate of change in PCB concentration using multiple linear regression, adjusting for preconception PCB concentration and lipids, centering each by its mean to facilitate inter- pretation of model intercepts as the change in PCBs at the average lipid and baseline PCB concentrations. Estimating the difference in PCBs’ grouping concentrations between the two measurements simplified the variance structure by removing part of the correlation between observations on the same woman. Additional covariates were not included in models given the absence of signiﬁcance, nor were separate models run for the three clinical pregnancy losses. Serum lipids could not be quantified for 10 women (8 live births, 1 pregnancy loss, and 1 infertile) given 1321