Bloom et al. insufficient sample necessitating expectation maximization (Yuan 2000). We evaluated the normality assumption for each regression model by examining resid- uals employing Q–Q plots and by comparing predicted values with those generated employ- ing the LOESS nonparametric regression procedure, because changes in PCB concentra- tions deviated substantially from normality even with transformation. Significance was deﬁned as p < 0.05 for two-tailed tests; crude differences in overall change and daily rate of change in PCB concentration were evaluated using the Wilcoxon signed rank test. Table 1. Description of study cohort at baseline by reproductive outcome [median (range)]. Prenatal (n = 48) Pregnancy loss (n = 10) Infertility (n = 9) Characteristic PCBs (ng/g serum) Total PCBs Estrogenic PCBs Antiestrogenic PCBs PCB-118 PCB-153 Total serum lipids (mg/dL) Age (years) BMI (kg/m2) Gravidity (no. of pregnancies) Parity (no. of births) Breast-feeding (no. of months)a PCB grouping Total Estrogenic Antiestrogenic PCB-118 PCB-153 *p < 0.05; **p < 0.0001. PCB grouping Total PCBs Estrogenic PCBs Antiestrogenic PCBs PCB-118 PCB-153 *p < 0.05; **p < 0.0001. Overall change (ng/g serum) Daily rate of change (ng/g serum) 1322 5.27 (3.66–12.68) 2.29 (1.66–3.77) 0.20 (0.03–0.65) 0.09 (0.01–0.31) 0.26 (0.10–1.24) 530.17 (357.57–915.38) 30.5 (26–34) 22.18 (17.7–39.9) 1 (0–5) 1 (0–3) 6.0 (1–28) 5.21 (3.89–9.09) 2.29 (1.78–4.54) 0.18 (0.03–0.33) 0.12 (0.02–0.16) 0.25 (0.16–0.47) 529.05 (307.91–829.94) 29.5 (26–33) 22.39 (17.54–34.56) 1 (0–3) 1 (0–3) 10.5 (2–13) Baseline measure with prenatal (n = 48) 0.328* 0.168 0.558** 0.755** 0.513* PCB grouping Total PCBs Estrogenic PCBs Antiestrogenic PCBs PCB-118 PCB-153 Total PCBs Estrogenic PCBs Antiestrogenic PCBs PCB-118 PCB-153 Baseline measure with early loss (n = 10) 0.745* 0.588 0.394 0.770* 0.770* Prenatala (n = 48) –1.012** –0.444** –0.106** –0.016 –0.021 –0.034** –0.016** –0.004** –0.001* –0.000 4.62 (4.10–7.19) 2.09 (1.79–3.18) 0.22 (0.12–0.29) 0.08 (0.05–0.21) 0.24 (0.15–0.55) 475.39 (414.45–723.67) 30.0 (26–32) 21.46 (17.28–31.15) 0.5 (0–2) 0 (0–2) 13.0 (6–20) PCB-118 0.341* 0.258* 0.799** 1.000 PCB 153 0.600** 0.499** 0.629** 0.418* 1.000 Baseline measure with infertility (n = 9) 0.317 0.383 0.467 0.300 0.567 Prenatal measure with postnatal (n = 47) –0.265 –0.346* 0.095 0.281 0.320* BMI, body mass index. None of the above differences achieved statistical signiﬁcance. Reproductive outcome refers to the timing of the second blood collection. aRestricted to the 39 women reporting a history of breast-feeding at baseline. Table 2. Spearman correlation coefﬁcients among baseline PCB groupings (n = 67). Total PCBs Estrogenic PCBs Antiestrogenic PCBs 1.000 0.938** 1.000 0.494** 0.343* 1.000 Table 3. Spearman correlation coefficients among paired biospecimens by PCB grouping and critical window. Table 4. Mean adjusted overall and daily rates of change in serum PCB concentrations, by PCB groupings and reproductive outcome. Pregnancy lossa (n = 10) Infertilitya Prenatal to postnatalb (n = 9) (n = 47) –1.452** –0.647** –0.093* –0.014 –0.041 –0.085** –0.040** –0.004# –0.001 –0.003 –0.281 –0.312 0.065 0.010 –0.015 –0.000 –0.001 0.000 0.000 –0.000 1.938* 0.628* 0.228* 0.023 0.016 0.008* 0.003* 0.001* 0.000## 0.000## aAdjusted for overall change (mg/dL serum) or daily rate of change (mg/dL serum) in total serum lipids and baseline PCB concentration (ng/g serum). bAdjusted for overall change or daily rate of change in total serum lipids (mg/dL), prenatal PCB concentration and duration of breast-feeding between delivery and the postnatal sample. *p ≤ 0.004; **p < 0.0001; #p = 0.03. ##Observed values were 0.000115 and 0.000053, respectively, but rounded to zero. VOLUME 115 | NUMBER 9 | September 2007 • Environmental Health Perspectives Results We observed no signiﬁcant differences in the median preconception PCB concentrations by PCB grouping and reproductive outcome or for any of the other study covariates (Table 1). The median interval between the timing of the two serum PCB measurements was 63 days [interquartile range (IQR) = 34] for early preg- nancy loss, 79 days (IQR = 76) for clinical pregnancy loss, 151 days (IQR = 180.5) for live birth, 446 days (IQR = 127) for infertility, and 240 days (IQR = 21) for the pre- and postnatal. Baseline PCB groupings were signifi- cantly correlated with each other (Table 2). Significant correlations were found between baseline and prenatal measures for all PCB groupings except estrogenic PCBs, between baseline and early pregnancy loss measures for total PCBs, PCB-118 and PCB-153, and between pre- and postnatal samples for estro- genic PCBs and PCB-153 (Table 3). Table 4 presents the adjusted regression models for overall and daily rate of change in concentrations by PCB grouping and repro- ductive outcome. p-Values are presented in lieu of 95% conﬁdence intervals to avoid the use of negative exponents, though the latter are avail- able on request. Signiﬁcant mean overall and daily rates of change in PCB concentrations (nanograms per gram serum) were consistently observed for women becoming pregnant regardless of outcome for total, estrogenic, and antiestrogenic groupings. Results for the infer- tile women were inconsistent (positive and negative) and none were signiﬁcant. Of partic- ular note is the absence of any overall or daily changes in concentrations for PCB-153 regard- less of reproductive outcome, in contrast to an observed signiﬁcant change (–0.001) for daily PCB-118 among women giving birth. Overall mean PCB concentrations (nanograms per gram serum) adjusted for duration of breast- feeding (reported by 34 women) signiﬁcantly increased in women from prenatal to postnatal measurement for total (1.938), estrogenic (0.628), and antiestrogenic PCBs (0.228) as did the daily rate of change—0.008, 0.003, and 0.001, respectively. Median serum lipids (expressed in milligrams per deciliter) declined 15.97 (range –282.89 to 261.29) from pre- conception to prenatal for women whose preg- nancies resulted in a birth, and 1.57 (range –144.71 to 105.96) for women experiencing losses, but increased 18.97 (range –53.78 to 137.68) for infertile women. None of these results achieved signiﬁcance (data not shown). Overall and daily rates of change in serum lipids were not signiﬁcant predictors of changes in PCB concentrations (data not shown), except for the daily rate of change in anti- estrogenic PCBs between preconception and the prenatal blood and between the preconcep- tion and EPL measurements (i.e., β = 0.0004 and β = 0.0009, respectively; p = 0.02).