Perfluorinated compounds (PFCs) are used as stain, grease and water repellents in a wide range of consumer products. Despite their widespread use and known thyroid disrupting potential in animal studies, uncertainties remain about sources of exposure and thyroid effects in humans. Thyroid effects are of particular concern during early pregnancy, when thyroid hormones play a critical role in fetal brain development. The Chemicals, Health and Pregnancy study (CHirP) was designed to address these knowledge gaps. The main goals of this dissertation were 1) to describe and evaluate a wide range of recruitment techniques used to enroll women in early pregnancy into the study, 2) to identify the main determinants of PFC levels in maternal serum, and 3) to examine the relationships between PFCs and thyroid hormones in maternal serum during a critical window of thyroid-mediated fetal brain development. One hundred and fifty two women from Metro Vancouver were recruited into the study. Posters, flyers, and a booth at pregnancy trade shows were among the most effective recruitment methods. The recruited population was older, less ethnically diverse, more educated and more affluent than the population of pregnant women in Vancouver (Chapter 2). Significant determinants of PFCs in maternal serum included pork-based foods, raw fish and shellfish, microwave and movie theatre popcorn, ethnicity, time spent in cars and airplanes, mattress age, stain repellent use on carpets, spot remover use on carpets, rugs and furniture, and levels of certain PFCs or their precursors in dust. Maternal PFC levels also declined strongly with parity, highlighting concerns about fetal or infant exposures to PFCs across the placenta or via breast milk (Chapter 3). We found significant negative relationships between several PFCs in maternal serum and maternal free thyroxine (fT4), and positive relationships with maternal thyroid stimulating hormone (TSH), but only in women with markers of autoimmune hypothyroidism. These results suggest that PFCs may exacerbate low fT4 and high TSH levels in up to 45,000 pregnancies per year in Canada (i.e. in the 10% of women with these markers), with unknown effects on fetal brain development (Chapter 4). These results await replication in larger, population-based studies.