268 whether individual congeners were considered, and was based on samples obtained after PCB use was regulated [6– 8]. The literature on the pesticide DDT and breast cancer is subject to similar limitations [9]. Findings on the relation of PCBs measured in middle age to breast cancer are complex and have been reviewed previously in detail (see Table 3, pp. 2872–2876 in Brody et al. [8]). Brieﬂy, some studies reported positive associa- tions with individual congeners, others reported no asso- ciations for total PCBs or any individual congeners, and others reported negative associations with some tumor types. The most consistent, positive ﬁndings were reported where interaction between exposure and CYP1A1 gene polymorphisms could be considered. Three studies found higher PCB exposures with postmenopausal breast cancer in combination with the CYP1A1 polymorphism, M2 type [10–12] and another reported this association with pre- menopausal breast cancer [13]. The present prospective study adds a unique perspective by testing the hypothesis that exposure to PCBs, measured during the early postpartum, is associated with increased risk of early breast cancer. Because of the long half-life of PCBs [14], early postpartum levels also reﬂect exposure during pregnancy [15] and possibly during childhood and adolescence. To our knowledge, this is the ﬁrst study to report on breast cancer associations in relation to measured PCBs levels during critical periods of vulnerability for the breast. Materials and methods Subjects Subjects were participants in the Child Health and Devel- opment Studies (CHDS), residents of the Oakland, Cali- fornia area and members of the Kaiser Permanente Health Plan who sought obstetric care between 1959 and 1967 [16]. Subjects voluntarily participated in the CHDS, giving an oral informed consent for an in-person interview, col- lection of blood specimens at several points in pregnancy and the early postpartum, and permission for medical record access. This study was reviewed and approved by The Institutional Review Board of the Public Health Institute and we have complied with all federal guidelines governing use of human participants. Breast cancer cases were identiﬁed by linkage to the California Cancer Registry, and the California Vital Status Records [17]. All names for each CHDS subject are sub- mitted for cancer linkages using ﬁxed (i.e. birth date, sex, race, and name) and changeable (i.e. address and patient record number) identiﬁers. A rigorous protocol is used to verify cases, comparing ﬁxed versus changeable identiﬁers 123 Breast Cancer Res Treat (2012) 136:267–275 by manual review. The California Cancer Registry is reported to be [99 % complete after a lag time of about 2 years [18]. Cases were deﬁned as women with incident invasive or non-invasive breast cancer diagnosed before age 50, or deaths due to breast cancer before age 50, obtained from linkage conducted in early 1998. There were 133 cases who met study criteria. All members of the CHDS cohort are additionally linked to the California Department of Motor Vehicles (DMV) ﬁles on a regular basis to determine res- idence history allowing us to assess their control status and to update any name changes. All names registered with the DMV are used in establishing a match. Simultaneous linkage of multiple family members enhances matching. The regular DMV matching provides a history of location for each subject which is used to determine the population at risk for cancer, corresponding with geographic surveil- lance by California’s cancer registries. Subjects who can- not be located are considered lost to follow-up at the date of their last deﬁnitive classiﬁcation as a California resident. One control, matched exactly on birth year, was selected at random for each case from those who were under cancer surveillance and known to be free of breast cancer at the age of diagnosis for the matching case. The median time to diagnosis for cases was 17 years. The mean age at diag- nosis was 43 years. Serum assays Exposure to PCBs was measured by assays of serum samples drawn during the early postpartum period, within 1–3 days of delivery. Postpartum samples were used to conserve valuable archived serum samples drawn in each trimester for future studies where timed samples are essential. PCBs have a long half-life and prior work has established stability of organochlorine levels assayed across all trimesters of pregnancy and the early postpartum [15]. For this reason, postpartum levels of PCBs may accurately rank women on their pregnancy exposure, covering two potential vulnerable periods of susceptibility of the breast to toxicants. PCBs were assayed in the laboratory of Dr. Mary Wolff [19] using modiﬁcations of methods developed previously by Brock et al. [20]. Brieﬂy, a polar extraction of serum lipids is followed by a column chromatographic clean-up and enrichment step, with analysis by gas chromatography with electron capture detection. Limits of detection were approximately 0.07 ng/mL for individual compounds based on three times the standard deviation of the levels found in the lowest quality control plasma pool [21]. When the serum pool and blanks were considered together [22], the limit of detection was 0.01–0.1 ng/mL; the instru- mental limit of detection based on a peak-to-noise ratio of