Boucher et al. at p < 0.05. Significant results that were asso­ ciated with a moderating variable were noted. The strengths and limitations of each study were considered in the cognitive neuropsy­ chological analysis of effects. Inconsistencies between cohorts thought to be attributable to unique influences associated with a specific study are noted. We included nine birth cohort studies on the effects of prenatal PCB exposure. Some particularities of these studies are summarized in Table 1. In all tables studies are presented in ascending order of severity of exposure. Because the laboratory procedures and types of specimens used for PCB exposure analysis varied across studies, the severity of exposure is estimated from the median PCB congener no. 153 (PCB1­53) levels in maternal serum, based on the work of Longnecker and col­ leagues (2003). Selected studies were con­ ducted between 1959 and 2008 in North America, Northern Europe, and Japan. The most frequent source of exposure was sea­ food consumption. In the following section we briefly describe these cohort studies in chronological order of publication. North Carolina. The North Carolina Breast Milk and Formula Project, initiated in 1978 in the Raleigh–Durham, North Carolina, USA, area was designed to evaluate the morbidity linked to PCB and dichloroethylene exposure in breast milk (Rogan et al. 1986a). PCB expo­ sure was assumed to come from background levels in the general food supply rather than from specific dietary habits. Biologic samples from cord and maternal blood, breast milk, and placenta were obtained. More than 800 children were tested at 6 and 12 months of age. Approximately 700 were reassessed at 18 and 24 months of age and again between 3 and 5 years of age (Gladen et al. 1988; Gladen and Rogan 1991). Michigan. More than 8,000 women who gave birth in one of four western Michigan hospitals in 1980–1981 were interviewed after delivery. Among them, women who had ingested ≥ 26 lbs (about 11.8 kg) or more of Lake Michigan fish in the 6 preceding years were invited to participate. The initial sample consisted of 313 women. Seventy­seven per ­ cent reported moderate or large quantities of lake fish ingestion, and the other 23% were randomly selected from among those who did not report any lake fish consumption (Jacobson et al. 1986). Breast milk and cord and maternal blood samples were collected. Children were tested at 5 and 7 months of age, then again when they were 4 and 11 years old. At the last follow­up, the remain ing sample size was 212. Faroe Islands. Faroe Islands residents are heavily exposed to PCBs and methylmercury (MeHg) because of their pilot whale blubber and meat intake and extensive fish consump­ tion. In 1986–1987, mother–child dyads (n = 1,022) were recruited to take part in a study on the effects of exposure to these contami­ nants. PCB exposure was estimated for 435 cord tissue samples. More than 400 children with documented PCB exposure were tested at 7 years of age (Grandjean et al. 2001). Germany. Between 1993 and 1995, 171 mother–child dyads were recruited in three Düsseldorf hospitals. Cord blood and maternal milk samples were collected to determine PCB exposure. Despite high PCB levels found, the source of exposure is not documented. Children were tested at 7, 18, 30, and 42 months of age. At the 42­month followu­p, the remaining number of children was 91 (Walkowiak et al. 2001). Netherlands. The Netherlands cohort study was performed in the Groningen and the Rotterdam areas, where consumption of dairy products and meat were the main sources of PCB exposure (Patandin et al. 1999a). Half the women recruited breast­ fed their child for at least 6 weeks; the other half preferred formula feeding. Cord blood, maternal blood, and breast milk samples were collected. Various dioxins and furans were ana­ lyzed in the biologic samples. Children were tested at 3, 7, 18, and 42 months of age, and again at 6.5 and 9 years. Of the 418 infants recruited at birth, data were available for 376 at 6.5 years of age (Vreugdenhil et al. 2002). The 9­ year assessment included 83 children from Rotterdam (Vreugdenhil et al. 2004a). Oswego. The Oswego Newborn and Infant Development Project is a prospective longitudi­ nal study conducted in the state of New York, USA. This study examined the behavioral effects of pre­ and perinatal exposure to PCBs and other persistent organic contaminants (Stewart et al. 2000). The consumption of contaminated Lake Ontario fish was the source of PCB expo­ sure. Cord blood, placenta, and breast milk samples were collected. MeHg, lead, dichloro­ ethylene, hexachlorobenzene, and mirex lev­ els were also documented. Child assessments were conducted at 6, 12, 38, and 54 months of age and again at 8 and 9.5 years of age. From the 309 initial participants, about 200 children were tested at the last followu­p phase (Stewart et al. 2008). Nunavik. Between November 1995 and March 2001, pregnant Inuit women from the three largest communities of the Hudson Bay coast (Northern Québec, Canada) were invited to participate in the Environmental Contaminants and Child Development Study. PCBs, other chlorinated pesticides, MeHg, lead, polyunsaturated fatty acids, and selenium levels were estimated from cord blood, maternal blood and hair, and breast milk samples (Muckle et al. 2001). More than 190 infants were tested at 6 and 12 months of age. At 66 months of age, between 1999 and 2001, an additional 110 children under­ went neuromotor and neurophysiologic test­ ing (Després et al. 2005; Saint­ Amour et al. 2006). These children were initially part of the Cord Blood Monitoring Program, aimed at documenting levels of exposure to environ­ mental contaminants in the Inuit population of Northern Québec (Muckle et al. 1998). The same contaminant and nutrient levels as those from the preceding study were analyzed from cord blood samples. Collaborative Perinatal Project. The Collaborative Perinatal Project (CPP) is a longitudinal study that followed the develop­ ment of > 50,000 children born between 1959 and 1966 in the United States. About 42,000 pregnant women were recruited in 12 hospi­ tals from 11 different cities across the country. Forty years later, data from this study were ovlume 117 | number 1 | January 2009 • Environmental Health Perspectives Table 1. PCB exposure in the different selected birth cohort studies. Cohorta Hokkaido Oswego North Carolina Netherlands Nunavik (1st) Michigan Germany CPP Faroe Islands in the present article). 8 Initial no. 135 309 859 418 175 313 171 1,207 435 Birth year 2002–2004 1991–1994 1978–1982 1990–1992 1995–1998 1980–1981 1993–1995 1959–1965 1986–1987 Source of exposure Fish Fish Unspecific Dairy products, meat Fish, marine mammals Fish Not specified Unspecific Fish, marine mammals Median PCB-153 (ng/g fat)b 23 40 80 100 100 120 140 140 450c Reference Nakajima et al. 2006 Darvill et al. 2000 Rogan et al. 1986b Patandin et al. 1999b Muckle et al. 2001 Schwartz et al. 1983 Winneke et al. 1998 Daniels et al. 2003 Grandjean et al. 2001 CPP, Collaborative Perinatal Project. aThe cohorts are presented in ascending order of median prenatal PCB-153 concentration. bEstimated median of PCB-153 levels in maternal serum (Longnecker et al. 2003; Nakajima et al. 2006). cThis value was estimated from 173 maternal serum specimens collected in the second Faroese birth cohort (children born 8 years after the first Faroese cohort considered