Bloom et al. pre- and postconception biospecimens with an additional biospecimen after an untoward reproductive outcome or birth for fertile women. Thus, we had only one blood sample obtained during pregnancy. We also recognize that some pregnancies may have been unde- tected by home pregnancy tests, possibly resulting in women being misclassified as infertile, despite the absence of signiﬁcant dif- ferences in baseline concentrations by repro- ductive outcome. Not all women’s blood specimens could be precisely collected on the same day, given that women resided in 16 counties in New York State with a single research nurse available for home blood collec- tion. Timing of the second sample also depended on the woman’s compliance with home pregnancy testing and her ability to accurately recognize and report a positive test. Given the educational attainment of the women coupled with instruction by the nurse with regard to fertility awareness and the proper use of home pregnancy test results, pregnancy recognition bias is not likely to have affected the timing of the test. Our analyses do consider the interval (in days) between our measurements. We decided a priori not to correct for multiple compar- isons given the exploratory nature of this work and our intent to try to globally assess patterns within and between PCB groupings and by reproductive outcomes. Finally, we remain uncertain about how best to model the chemi- cal mixtures more representative of women’s actual exposures. Our initial attempt to categorize PCB congeners in an a priori man- ner should be viewed as preliminary and need- ing further reﬁnement as our understanding of the biological activity and underlying mecha- nisms evolves for this class of compounds. 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Multiple Imputation for Missing Data: Concepts and New Development. Rockville, MD:SAS Institute Inc. 1324 VOLUME 115 | NUMBER 9 | September 2007 • Environmental Health Perspectives